Index - MDN Web Docs Glossary: Definitions of Web-related terms 编辑

Found 532 pages:

#	Page	Tags and summary
1	MDN Web Docs Glossary: Definitions of Web-related terms	Beginner, Definitions, Dictionary, Glossary, Index, Landing, Terminology
1	MDN Web Docs Glossary: Definitions of Web-related terms	Web technologies contain long lists of jargon and abbreviations that are used in documentation and coding. This glossary provides definitions of words and abbreviations you need to know to successfully understand and build for the web.
2	404	Glossary, HTTP Errors, Infrastructure, Navigation
2	404	A 404 is a Standard Response Code meaning that the Server cannot find the requested resource.
3	502	502, Bad Gateway, Glossary, HTTP Errors, Infrastructure, Navigation
3	502	An HTTP error code meaning "Bad Gateway".
4	Abstraction	Abstraction, Coding, CodingScripting, Glossary, Programming Language
4	Abstraction	Abstraction in computer programming is a way to reduce complexity and allow efficient design and implementation in complex software systems. It hides the technical complexity of systems behind simpler API.
5	Accessibility tree	AOM, Accessibility, DOM, Glossary, Reference
5	Accessibility tree	The accessibility tree contains accessibility-related information for most HTML elements.
6	Accessibility	Accessibility, Glossary
6	Accessibility	Web Accessibility (A11Y) refers to best practices for keeping a website usable despite physical and technical restrictions. Web accessibility is formally defined and discussed at the W3C through the WAI (WAI).
7	Adobe Flash	Adobe, CodingScripting, Flash, Glossary, Infrastructure
7	Adobe Flash	Flash is a deprecated technology developed by Adobe for viewing expressive web applications, multimedia content, and streaming media. Adobe Flash can run from supporting Browser via a browser plug-in.
8	Ajax	AJAX, CodingScripting, Glossary, Infrastructure, l10n:priority
8	Ajax	Ajax, which initially stood for Asynchronous JavaScript And XML, is a programming practice of building complex, dynamic webpages using a technology known as XHR_(XMLHttpRequest).
9	Algorithm	CodingScripting, Glossary
9	Algorithm	An algorithm is a self-contained series of instructions to perform a function.
10	Alignment container	Alignment container, CSS, Glossary, alignment
10	Alignment container	The alignment container is the rectangle that the alignment subject is aligned within. This is defined by the layout mode; it is usually the alignment subject’s containing block, and assumes the writing mode of the box establishing the containing block.
11	Alignment subject	Alignment subject, CSS, Glossary, alignment
11	Alignment subject	In CSS Box Alignment the alignment subject is the thing (or things) being aligned by the property.
12	Alpha (alpha channel)	Alpha, Alpha Channel, Drawing, Glossary, Graphics, Translucency, Translucent, Transparency, Transparent, WebGL, WebXR, channel, color, pixel
12	Alpha (alpha channel)	Colors are represented in digital form as a collection of numbers, each representing the strength or intensity level of a given component of the color. Each of these components is called a channel. In a typical image file, the color channels describe how much red, green, and blue are used to make up the final color. To represent a color through which the background can be seen to some extent, a fourth channel is added to the color: the alpha channel. The alpha channel specifies how opaque the color is.
13	ALPN	ALPN, Draft, Glossary, NeedsContent, TLS
13	ALPN	Application-Layer Protocol Negotiation (ALPN) is a TLS extension which indicates what application layer protocol is negotiating the encrypted connection without requiring additional round trips.
14	API	CodingScripting, Glossary, Infrastructure
14	API	An API (Application Programming Interface) is a set of features and rules that exist inside a software program (the application) enabling interaction with it through software - as opposed to a human user interface. The API can be seen as a simple contract (the interface) between the application offering it and other items, such as third party software or hardware.
15	Apple Safari	Glossary, Navigation, WebMechanics
15	Apple Safari	Safari is a Browser developed by Apple and bundled with both macOS and iOS. It's based on the open source WebKit engine.
16	Application Context	CodingScripting, Glossary
16	Application Context	An application context is a top-level browsing context that has a manifest applied to it.
17	Argument	CodingScripting, Glossary, JavaScript
17	Argument	An argument is a value (primitive or object) passed as input to a function.
18	ARIA	Accessibility, Glossary
18	ARIA	ARIA (Accessible Rich Internet Applications) is a W3C specification for adding semantics and other metadata to HTML to cater to users of assistive technology.
19	ARPA	Glossary, Infrastructure
19	ARPA	.arpa (address and routing parameter area) is a TLD used for Internet infrastructure purposes, especially reverse DNS lookup (i.e., find the domain name for a given IP address).
20	Arpanet	Glossary, Infrastructure
20	Arpanet	The ARPAnet (advanced research projects agency network) was an early computer network, constructed in 1969 as a robust medium to transmit sensitive military data and to connect leading research groups throughout the United States. ARPAnet first ran NCP (network control protocol) and subsequently the first version of the Internet protocol or TCP/IPv6 suite, making ARPAnet a prominent part of the nascent Internet. ARPAnet was closed in early 1990.
21	Array	Array, CodingScripting, Glossary, JavaScript, programming
21	Array	An array is an ordered collection of data (either primitive or object depending upon the language). Arrays are used to store multiple values in a single variable. This is compared to a variable that can store only one value.
22	ASCII	Glossary, Infrastructure
22	ASCII	ASCII (American Standard Code for Information Interchange) is one of the most popular coding method used by computers for converting letters, numbers, punctuation and control codes into digital form. Since 2007, UTF-8 superseded it on the Web.
23	Asynchronous	Glossary, Web, WebMechanics, asynchronous
23	Asynchronous	The term asynchronous refers to
24	ATAG	ATAG, Accessibility, Authoring Tool Accessibility Guidelines, Glossary
24	ATAG	ATAG (Authoring Tool Accessibility Guidelines) is a W3C recommendation for building accessible-authoring tools that produce accessible contents.
25	Attribute	CodingScripting, Glossary, HTML
25	Attribute	An attribute extends an element, changing its behavior or providing metadata.
26	Bandwidth	Glossary, Infrastructure
26	Bandwidth	Bandwidth is the measure of how much information can pass through a data connection in a given amount of time. It is usually measured in multiples of bits-per-second (bps), for example megabits-per-second (Mbps) or gigabits-per-second (Gbps).
27	Base64	Advanced, Base64, JavaScript, Typed Arrays, URI, URL, Unicode Problem, atob(), btoa()
27	Base64	Base64 is a group of similar binary-to-text encoding schemes that represent binary data in an ASCII string format by translating it into a radix-64 representation. The term Base64 originates from a specific MIME content transfer encoding.
28	Baseline	CSS, Glossary, SVG, alignment, typography
28	Baseline	The baseline is a term used in European and West Asian typography meaning an imaginary line upon which the characters of a font rest.
29	beacon	Beacon, Glossary, Reference, Web Performance
29	beacon	A web beacon is a small object, such as a 1 pixel gif, embedded in markup, used to communicate information back to the web server or to 3rd party servers. Beacons are generally included to provide information about the user for statistical purposes. Beacons are often included within third party scripts for collecting user data, performance metrics and error reporting.
30	Bézier curve	Bézier curve, Glossary, Graphics, Reference
30	Bézier curve	A Bézier curve (pronounced [bezje]) is a mathematically described curve used in computer graphics and animation. In vector image, they are used to model smooth curves that can be scaled indefinitely.
31	BiDi	Accessibility, Glossary
31	BiDi	BiDi (BiDirectional) refers to a document containing both right-to-left and left-to-right text. Even when both directionalities occur in the same paragraph, the text in each language must appear in its proper directionality.
32	BigInt	BigInt, Glossary, JavaScript, Reference, arbitrary precision format
32	BigInt	In JavaScript, BigInt is a numeric data type that can represent integers in the arbitrary precision format. In other programming languages different numeric types can exist, for examples: Integers, Floats, Doubles, or Bignums.
33	Blink	Glossary, Infrastructure, Layout, Rendering engine
33	Blink	Blink is an open-source browser layout engine developed by Google as part of Chromium (and therefore part of Google Chrome as well). Specifically, Blink began as a fork of the WebCore library in WebKit, which handles layout, rendering, and DOM, but now stands on its own as a separate rendering engine.
34	Block cipher mode of operation	Block cipher mode of operation, Cryptography, Glossary, Security
34	Block cipher mode of operation	A block cipher mode of operation, usually just called a "mode" in context, specifies how a block cipher should be used to encrypt or decrypt messages that are longer than the block size.
35	Block	Disambiguation, Glossary
35	Block	The term block can have several meanings depending on the context. It may refer to:
36	Block (CSS)	CSS, CodingScripting, Design, Glossary, HTML
36	Block (CSS)	A block on a webpage is an HTML element that appears on a new line, i.e. underneath the preceding element in a horizontal writing mode, and above the following element (commonly known as a block-level element). For example, `p` is by default a block-level element, whereas `a` is an inline element — you can put several links next to one another in your HTML source and they will sit on the same line as one another in the rendered output.
37	Block (scripting)	CodingScripting, Glossary, JavaScript
37	Block (scripting)	In JavaScript, a block is a collection of related statement enclosed in braces ("{}"). For example, you can put a block of statements after an `Statements/if...else` block, indicating that the interpreter should run the code inside the block if the condition is true, or skip the whole block if the condition is false.
38	Boolean	Boolean, CodingScripting, Glossary, JavaScript, Programming Languages, data types
38	Boolean	In computer science, a Boolean is a logical data type that can have only the values `true` or `false`.
39	Boot2Gecko	B2G, Boot2Gecko, Firefox OS, Glossary, Infrastructure, Intro
39	Boot2Gecko	Boot2Gecko (B2G) is the engineering codename for Firefox OS and refers to builds that haven't yet received official Firefox OS branding. (Firefox OS was also often called Boot2Gecko before the project had an official name.)
40	Bootstrap	Bootstrap, CSS, Glossary, Intro, framework
40	Bootstrap	Bootstrap is a free, open source HTML, CSS, and JavaScript framework for quickly building responsive websites.
41	Bounding Box	Bounding Box, CodingScripting, Design, Glossary
41	Bounding Box	The bounding box of an element is the smallest possible rectangle (aligned with the axes of that element's user coordinate system) that entirely encloses it and its descendants.
42	Breadcrumb	Accessibility, Glossary, Navigation, Search, Site map, breadcrumb
42	Breadcrumb	A breadcrumb, or breadcrumb trail, is a navigational aid that is typically placed between a site's header and the main content, displaying either a hierarchy of the current page in relation to the site's structure, from top level to current page, or a list of the links the user followed to get to the current page, in the order visited.
43	Brotli	Brotli, Glossary, Reference, Web Performance, compression
43	Brotli	Brotli is a general-purpose lossless compression algorithm.
44	Browser	Glossary, Navigation
44	Browser	A Web browser or browser is a program that retrieves and displays pages from the World Wide Web, and lets users access further pages through hyperlink. A browser is the most familiar type of user agent.
45	Browsing context	Glossary
45	Browsing context	A browsing context is the environment a browser displays a `Document`. In modern browsers, it usually is a tab, but can be a window or even only parts of a page, like a frame or an iframe.
46	buffer	Buffer, CodingScripting, Glossary, NeedsContent
46	buffer	A buffer is a storage in physical memory used to temporarily store data while it is being transferred from one place to another.
47	Cache	Glossary, HTTP
47	Cache	A cache (web cache or HTTP cache) is a component that stores HTTP responses temporarily so that it can be used for subsequent HTTP requests as long as it meets certain conditions.
48	Cacheable	Glossary, WebMechanics
48	Cacheable	A cacheable response is an HTTP response that can be cached, that is stored to be retrieved and used later, saving a new request to the server. Not all HTTP responses can be cached, these are the following constraints for an HTTP response to be cached:
49	CalDAV	CalDAV, Glossary, Infrastructure
49	CalDAV	CalDAV (Calendaring extensions to WebDAV) is a protocol standardized by the IETF and used to remotely access calendar data from a server.
50	Call stack	Call Stack, CodingScripting, Glossary, JavaScript
50	Call stack	A call stack is a mechanism for an interpreter (like the JavaScript interpreter in a web browser) to keep track of its place in a script that calls multiple function — what function is currently being run and what functions are called from within that function, etc.
51	Callback function	Callback, Callback function, CodingScripting, Glossary
51	Callback function	A callback function is a function passed into another function as an argument, which is then invoked inside the outer function to complete some kind of routine or action.
52	Canonical order	Canonical order, CodingScripting, Glossary
52	Canonical order	In CSS, canonical order is used to refer to the order in which separate values need to be specified (or parse) or are to be serialization as part of a CSS property value. It is defined by the formal syntax of the property and normally refers to the order in which longhand values should be specified as part of a single shorthand value.
53	Canvas	CodingScripting, Glossary, Graphics, HTML, JavaScript
53	Canvas	The canvas element is part of HTML5 and allows for dynamic, scriptable rendering of 2D and 3D shapes and bitmap images.
54	Card sorting	Card sorting, Design, Glossary
54	Card sorting	Card sorting is a simple technique used in Information architecture whereby people involved in the design of a website (or other type of product) are invited to write down the content / services / features they feel the product should contain, and then organize those features into categories or groupings. This can be used for example to work out what should go on each page of a website. The name comes from the fact that often card sorting is carried out by literally writing the items to sort onto cards, and then arranging the cards into piles.
55	CardDAV	CardDAV, Glossary, Infrastructure
55	CardDAV	CardDAV (vCard Extension to WebDAV) is a protocol standardized by the IETF and used to remote-access or share contact information over a server.
56	caret	Cursor, Glossary, Input, caret, insertion point, text cursor, text entry, text input, text insertion point
56	caret	A caret (sometimes called a "text cursor") is an indicator displayed on the screen to indicate where text input will be inserted.
57	CDN	Glossary, Infrastructure
57	CDN	A CDN (Content Delivery Network) is a group of servers spread out over many locations. These servers store duplicate copies of data so that servers can fulfill data requests based on which servers are closest to the respective end-users. CDNs make for fast service less affected by high traffic.
58	Certificate authority	Cryptography, Glossary, Security
58	Certificate authority	A certificate authority (CA) is an organization that Signature/Security Digital certificate and their associated Key, thereby asserting that the contained information and keys are correct.
59	Certified	Apps, B2G, Firefox OS, Glossary, Security, Trustworthy
59	Certified	Certified means that an application, content or data transmission has successfully undergone evaluation by professionals with expertise in the relevant field, thereby indicating completeness, security and trustworthiness.
60	Challenge-response authentication	Security
60	Challenge-response authentication	In security protocols, a challenge is some data sent to the client by the server in order to generate a different response each time. Challenge-response protocols are one way to fight against replay attacks where an attacker listens to the previous messages and resends them at a later time to get the same credentials as the original message.
61	Character encoding	Composing, Glossary
61	Character encoding	An encoding defines a mapping between bytes and text. A sequence of bytes allows for different textual interpretations. By specifying a particular encoding (such as UTF-8), we specify how the sequence of bytes is to be interpreted.
62	Character set	Glossary, character encoding, character set
62	Character set	A character set is an encoding system to let computers know how to recognize Character, including letters, numbers, punctuation marks, and whitespace.
63	Character	CodingScripting, Glossary, strings
63	Character	A character is either a symbol (letters, numbers, punctuation) or non-printing "control" (e.g., carriage return or soft hyphen). UTF-8 is the most common character set and includes the graphemes of the most popular human languages.
64	Chrome	Browser, Chrome, Glossary, WebMechanics
64	Chrome	In a browser, the chrome is any visible aspect of a browser aside from the webpages themselves (e.g., toolbars, menu bar, tabs). This is not to be confused with the Google Chrome browser.
65	CIA	Glossary, Security
65	CIA	CIA (Confidentiality, Integrity, Availability) (also called the CIA triad or AIC triad) is a model that guides an organization's policies for information security.
66	Cipher suite	Cryptography, Glossary, Security
66	Cipher suite	A cipher suite is a combination of a key exchange algorithm, authentication method, bulk encryption cipher, and message authentication code.
67	Cipher	Cryptography, Glossary, Privacy, Security
67	Cipher	In cryptography, a cipher is an algorithm that can encryption cleartext to make it unreadable, and to decryption it back.
68	Ciphertext	Cryptography, Glossary, Privacy, Security
68	Ciphertext	In cryptography, a ciphertext is a scrambled message that conveys information but is not legible unless decryption with the right cipher and the right secret (usually a key), reproducing the original cleartext. A ciphertext's security, and therefore the secrecy of the contained information, depends on using a secure cipher and keeping the key secret.
69	Class	CodingScripting, Glossary
69	Class	In OOP, a class defines an object characteristics. Class is a template definition of an object's property and method, the "blueprint" from which other more specific instances of the object are drawn.
70	Client hints	Client hints, Glossary, Performance, Reference, Web Performance
70	Client hints	Client Hints are a set of HTTP request header fields for proactive content negotiation allowing clients to indicate a list of device and agent specific preferences.
71	Closure	CodingScripting, Glossary
71	Closure	The binding which defines the scope of execution. In JavaScript, function create a closure context.
72	CMS	CMS, Composing, Content management system, Glossary
72	CMS	A CMS (Content Management System) is software that allows users to publish, organize, change, or remove various kinds of content, not only text but also embedded images, video, audio, and interactive code.
73	Code splitting	Glossary, Reference, Web Performance, code splitting, latency
73	Code splitting	Code splitting is the splitting of code into various bundles or components which can then be loaded on demand or in parallel.
74	Codec	Glossary, WebMechanics
74	Codec	A codec (a blend word derived from "coder-decoder") is a program, algorithm, or device that encodes or decodes a data stream. A given codec knows how to handle a specific encoding or compression technology.
75	Compile time	CodingScripting, Glossary, JavaScript
75	Compile time	The compile time is the time from when the program is first loaded until the program is parse.
76	Compile	CodingScripting, Glossary
76	Compile	Compiling is the process of transforming a computer program written in a given computer programming into an equivalent program of another language. A compiler is a software to execute this task. Sometimes, this task is also referred to as "assembling" or "build", which typically indiciates more than just compilation is done, e.g. packaging it in a binary format.
77	Computer Programming	CodingScripting, Computer Programming, Programming Language, programming
77	Computer Programming	Computer programming is a process of composing and organizing a collection of instructions. These tell a computer/software program what to do in a language which the computer understands. These instructions come in the form of many different languages such as C++, Java, JavaScript, HTML, Python, Ruby, and Rust.
78	Conditional	Beginner, CodingScripting, Glossary
78	Conditional	A condition is a set of rules that can interrupt normal code execution or change it, depending on whether the condition is completed or not.
79	Constant	CodingScripting, Constant, Glossary
79	Constant	A constant is a value that the programmer cannot change, for example numbers (1, 2, 42). With variable, on the other hand, the programmer can assign a new value to a variable name already in use.
80	Constructor	CodingScripting, Glossary
80	Constructor	A constructor belongs to a particular class object that is instantiated. The constructor initializes this object and can provide access to its private information. The concept of a constructor can be applied to most OOP languages. Essentially, a constructor in JavaScript is usually declared at the instance of a class.
81	Continuous Media	Glossary, Media
81	Continuous Media	Continuous media is data where there is a timing relationship between source and destination. The most common examples of continuous media are audio and motion video. Continuous media can be real-time (interactive), where there is a "tight" timing relationship between source and sink, or streaming (playback), where the relationship is less strict.
82	Control flow	CodingScripting, Glossary, JavaScript
82	Control flow	The control flow is the order in which the computer executes statements in a script.
83	Cookie	Glossary, WebMechanics
83	Cookie	A cookie is a small piece of information left on a visitor's computer by a website, via a web browser.
84	Copyleft	Glossary, OpenPractices, Remixing, Sharing
84	Copyleft	Copyleft is a term, usually referring to a license, used to indicate that such license requires that redistribution of said work is subject to the same license as the original. Examples of copyleft licenses are the GNU GPL (for software) and the Creative Commons SA (Share Alike) licenses (for works of art).
85	CORS-safelisted request header	CORS, Fetch
85	CORS-safelisted request header	A CORS-safelisted request header is one of the following HTTP headers:
86	CORS-safelisted response header	CORS, Fetch, Glossary, HTTP
86	CORS-safelisted response header	A CORS-safelisted response header is an HTTP header which has been safelisted so that it will not be filtered when responses are processed by CORS, since they're considered safe (as the headers listed in `Access-Control-Expose-Headers`).
87	CORS	Glossary, Infrastructure, Security
87	CORS	CORS (Cross-Origin Resource Sharing) is a system, consisting of transmitting Header, that determines whether browsers block frontend JavaScript code from accessing responses for cross-origin requests.
88	Crawler	Browser, Crawler, Glossary, Infrastructure
88	Crawler	A web crawler is a program, often called a bot or robot, which systematically browses the World Wide Web to collect data from webpages. Typically search engines (e.g. Google, Bing, etc.) use crawlers to build indexes.
89	CRLF	CR, CRLF, Glossary, Infrastructure, LF, carriage return, line feed
89	CRLF	CR and LF are control characters or bytecode that can be used to mark a line break in a text file.
90	Cross Axis	CSS, Glossary, cross axis, flexbox
90	Cross Axis	The cross axis in flexbox runs perpendicular to the main axis, therefore if your `flex-direction` is either `row` or `row-reverse` then the cross axis runs down the columns.
91	Cross-site scripting	Cross Site Scripting, DOM, Glossary, Security, XSS, exploit
91	Cross-site scripting	Cross-site scripting (XSS) is a security exploit which allows an attacker to inject into a website malicious client-side code. This code is executed by the victims and lets the attackers bypass access controls and impersonate users. According to the Open Web Application Security Project, XSS was the seventh most common Web app vulnerability in 2017.
92	CRUD	Glossary, Infrastructure
92	CRUD	CRUD (Create, Read, Update, Delete) is an acronym for ways one can operate on stored data. It is a mnemonic for the four basic functions of persistent storage. CRUD typically refers to operations performed in a database or datastore, but it can also apply to higher level functions of an application such as soft deletes where data is not actually deleted but marked as deleted via a status.
93	Cryptanalysis	Cryptography, Glossary, Privacy, Security
93	Cryptanalysis	Cryptanalysis is the branch of cryptography that studies how to break codes and cryptosystems. Cryptanalysis creates techniques to break cipher, in particular by methods more efficient than a brute-force search. In addition to traditional methods like frequency analysis and index of coincidence, cryptanalysis includes more recent methods, like linear cryptanalysis or differential cryptanalysis, that can break more advanced ciphers.
94	Cryptographic hash function	Cryptography, Glossary, Security
94	Cryptographic hash function	A cryptographic hash function, also sometimes called a digest function, is a cryptography primitive transforming a message of arbitrary size into a message of fixed size, called a digest. Cryptographic hash functions are used for authentication, digital signature, and HMAC.
95	Cryptography	Cryptography, Glossary, Privacy, Security
95	Cryptography	Cryptography, or cryptology, is the science that studies how to encode and transmit messages securely. Cryptography designs and studies algorithms used to encode and decode messages in an insecure environment, and their applications.
96	CSP	Glossary, HTTP, Infrastructure
96	CSP	A CSP (Content Security Policy) is used to detect and mitigate certain types of website related attacks like XSS and data injections.
97	CSRF	Glossary, Security
97	CSRF	CSRF (Cross-Site Request Forgery) is an attack that impersonates a trusted user and sends a website unwanted commands.
98	CSS pixel	CSS, CSS Pixel, Glossary, height, length, pixel, size, unit, width
98	CSS pixel	The CSS pixel—denoted in CSS with the suffix `px`—is a unit of length which roughly corresponds to the width or height of a single dot that can be comfortably seen by the human eye without strain, but is otherwise as small as possible. By definition, this is the physical size of a single pixel at a pixel density of 96 DPI, located an arm's length away from the viewer's eyes.
99	CSS preprocessor	CSS, Glossary
99	CSS preprocessor	A CSS preprocessor is a program that lets you generate CSS from the preprocessor's own unique syntax.
100	Selector (CSS)	CSS, CSS Selector, CodingScripting, Glossary, HTML, Selector
100	Selector (CSS)	A CSS selector is the part of a CSS rule that describes what elements in a document the rule will match. The matching elements will have the rule's specified style applied to them.
101	CSS	CSS, CodingScripting, Glossary, Web, l10n:priority
101	CSS	CSS (Cascading Style Sheets) is a declarative language that controls how webpages look in the browser.
102	CSS Object Model (CSSOM)	CSS, CSSOM, DOM, Glossary, Web Performance
102	CSS Object Model (CSSOM)	The CSS Object Model (CSSOM) is a map of all CSS selectors and relevant properties for each selector in the form of a tree, with a root node, sibling, descendant, child, and other relationship. The CSSOM is very similar to the DOM. Both of them are part of the critical rendering path which is a series of steps that must happen to properly render a website.
103	Data structure	CodingScripting, Data structure, Glossary
103	Data structure	Data structure is a particular way of organizing data so that it can be used efficiently.
104	Database	Database, Glossary, Sql
104	Database	A database is a storing system that collects organized data, to make some works easier like searching, structure, and extend.
105	Decryption	Cryptography, Glossary, Privacy, Security
105	Decryption	In cryptography, decryption is the conversion of ciphertext into cleartext.
106	Delta	Delta, Glossary, difference, value
106	Delta	The term delta refers to the difference between two values or states.
107	Denial of Service	Attack, Denial of Service, Glossary, Intro, Security
107	Denial of Service	No summary!
108	Descriptor (CSS)	CSS, CodingScripting, Glossary, NeedsContent
108	Descriptor (CSS)	A CSS descriptor defines the characteristics of an `at-rule`. At-rules may have one or multiple descriptors. Each descriptor has:
109	Deserialization	Deserialization, Deserialize, Glossary, JavaScript
109	Deserialization	The process whereby a lower-level format (e.g. that has been transferred over a network, or stored in a data store) is translated into a readable object or other data structure.
110	Developer Tools	CodingScripting, Developer Tools, Glossary
110	Developer Tools	Developer tools (or "development tools" or short "DevTools") are programs that allow a developer to create, test and debug software.
111	DHTML	CodingScripting, DHTML, Glossary, HTML
111	DHTML	DHTML (Dynamic HTML) refers to the code behind interactive webpages that need no plugins like Adobe Flash or Java. DHTML aggregates the combined functionality of HTML, CSS, the DOM, and JavaScript.
112	Digest	Cryptography, Glossary, Privacy, Security
112	Digest	A digest is a small value generated by a hash function from a whole message. Ideally, a digest is quick to calculate, irreversible, and unpredictable, and therefore indicates whether someone has tampered with a given message.
113	Digital certificate	Cryptography, Glossary, Security
113	Digital certificate	A digital certificate is a data file that binds a publicly known Key to an organization.
114	Distributed Denial of Service	Attack, DDoS, Denial of Service, Glossary, Intro, Security
114	Distributed Denial of Service	A Distributed Denial-of-Service (DDoS) is an attack in which many compromised systems are made to attack a single target, in order to swamp server resources and block legitimate users.
115	DMZ	Glossary, Networking, Security
115	DMZ	A DMZ is a way to provide an insulated secure interface between an internal network (corporate or private) and the outside untrusted world — usually the Internet.
116	DNS	DNS, Domain Name System, Glossary, Infrastructure
116	DNS	DNS (Domain Name System) is a hierarchical and decentralized naming system for Internet connected resources. DNS maintains a list of domain name along with the resources, such as IP addresses, that are associated with them.
117	Doctype	Browser, CodingScripting, DOCTYPE, Glossary, HTML, Intro
117	Doctype	In HTML, the doctype is the required "`<!DOCTYPE html>`" preamble found at the top of all documents. Its sole purpose is to prevent a browser from switching into so-called “quirks mode” when rendering a document; that is, the "`<!DOCTYPE html>`" doctype ensures that the browser makes a best-effort attempt at following the relevant specifications, rather than using a different rendering mode that is incompatible with some specifications.
118	Document directive	CSP, Directive, Document, Glossary, HTTP, Security
118	Document directive	CSP document directives are used in a `Content-Security-Policy` header and govern the properties of a document or worker environment to which a policy applies.
119	document environment	CodingScripting, Glossary, JavaScript
119	document environment	When the JavaScript global environment is a window or an iframe, it is called a document environment. A global environment is an environment that doesn't have an outer environment.
120	DOM (Document Object Model)	CodingScripting, DOM, Glossary
120	DOM (Document Object Model)	The DOM (Document Object Model) is an API that represents and interacts with any HTML or XML document. The DOM is a document model loaded in the browser and representing the document as a node tree, where each node represents part of the document (e.g. an element, text string, or comment).
121	Domain name	Domain Name, Glossary, Protocol, WebMechanics
121	Domain name	A domain name is a website's address on the Internet. Domain names are used in URL to identify to which server belong a specific webpage. The domain name consists of a hierarchial sequence of names (labels) separated by periods (dots) and ending with an TLD.
122	Domain sharding	DNS, Glossary, HTTP, Reference, Web Performance, latency
122	Domain sharding	Browsers limit the number of active connections for each domain. To enable concurrent downloads of assets exceeding that limit, domain sharding splits content across multiple subdomains. When multiple domains are used to serve multiple assets, browsers are able to download more resources simultaneously, resulting in a faster page load time and improved user experience.
123	Domain	Browser, Domain, Glossary, Infrastructure, Networking
123	Domain	A domain is an authority within the internet that controls its own resources. Its "domain name" is a way to address this authority as part of the hierarchy in a URL - usually the most memorable part of it, for instance a brand name.
124	Dominator	CodingScripting, Glossary
124	Dominator	In graph theory, node A dominates node B if every path from the root node to B passes through A.
125	DoS attack	Glossary, Security
125	DoS attack	DoS (Denial of Service) is a network attack that prevents legitimate use of server resources by flooding the server with requests.
126	DTLS (Datagram Transport Layer Security)	DTLS, Intro
126	DTLS (Datagram Transport Layer Security)	Datagram Transport Layer Security (DTLS) is a protocol used to secure datagram-based communications. It's based on the stream-focused Transport Layer Security (TLS), providing a similar level of security. As a datagram protocol, DTLS doesn't guarantee the order of message delivery, or even that messages will be delivered at all. However, DTLS gains the benefits of datagram protocols, too; in particular, the lower overhead and reduced latency.
127	DTMF (Dual-Tone Multi-Frequency signaling)	DTMF, Glossary, WebRTC
127	DTMF (Dual-Tone Multi-Frequency signaling)	Dual-Tone Multi-Frequency (DTMF) signaling is a system by which audible tones are used to represent buttons being pressed on a keypad. Frequently referred to in the United States as "touch tone" (after the Touch-Tone trademark used when the transition from pulse dialing to DTMF began), DTMF makes it possible to signal the digits 0-9 as well as the letters "A" through "D" and the symbols "#" and "*". Few telephone keypads include the letters, which are typically used for control signaling by the telephone network.
128	Dynamic programming language	CodingScripting, Glossary
128	Dynamic programming language	A dynamic programming language is a programming language in which operations otherwise done at compile-time can be done at run-time. For example, in JavaScript it is possible to change the type of a variable or add new properties or methods to an object while the program is running.
129	Dynamic typing	CodingScripting, Glossary, ProgrammingLanguage
129	Dynamic typing	Dynamically-typed languages are those (like JavaScript) where the interpreter assigns variable a type at runtime based on the variable's value at the time.
130	ECMA	Glossary, Stub, WebMechanics
130	ECMA	Ecma International (formally European Computer Manufacturers Association) is a non-profit organization that develops standards in computer hardware, communications, and programming languages.
131	ECMAScript	Glossary, WebMechanics
131	ECMAScript	ECMAScript is a scripting language specification on which JavaScript is based. Ecma International is in charge of standardizing ECMAScript.
132	Effective connection type	Glossary, Navigator, Network Information API, NetworkInformation, Performance, Reference, Web Performance, connect, effective connection type, effectiveType
132	Effective connection type	Effective connection type (ECT) refers to the measured network performance, returning a cellular connection type, like 3G, even if the actual connection is tethered broadband or WiFi, based on the time between the browser requesting a page and effective type of the connection.
133	Element	CodingScripting, Glossary, HTML
133	Element	An element is a part of a webpage. In XML and HTML, an element may contain a data item or a chunk of text or an image, or perhaps nothing. A typical element includes an opening tag with some attribute, enclosed text content, and a closing tag.
134	Empty element	CodingScripting, Glossary, Intermediate
134	Empty element	An empty element is an element from HTML, SVG, or MathML that cannot have any child nodes (i.e., nested elements or text nodes).
135	Encapsulation	CodingScripting, Glossary
135	Encapsulation	Encapsulation is the packing of data and function into one component (for example, a class) and then controlling access to that component to make a "blackbox" out of the object. Because of this, a user of that class only needs to know its interface (that is, the data and functions exposed outside the class), not the hidden implementation.
136	Encryption	Cryptography, Glossary, Privacy, Security
136	Encryption	In cryptography, encryption is the conversion of cleartext into a coded text or ciphertext. A ciphertext is intended to be unreadable by unauthorized readers.
137	Endianness	Coding, CodingScripting, Glossary
137	Endianness	Endian and endianness (or "byte-order") describe how computers organize the bytes that make up numbers.
138	Engine	CodingScripting, Glossary, NeedsContent
138	Engine	The JavaScript engine is an interpreter that parses and executes a JavaScript program.
139	Entity header	Glossary, WebMechanics
139	Entity header	An entity header is an header that describes the payload of an HTTP message (i.e. metadata about the message body). Entity headers include: `Content-Length`, `Content-Language`, `Content-Encoding`, `Content-Type`, `Expires`, etc. Entity headers may be present in both HTTP request and response messages.
140	Entity	CodingScripting, Composing, Glossary, HTML
140	Entity	An HTML entity is a piece of text ("string") that begins with an ampersand (`&`) and ends with a semicolon (`;`) . Entities are frequently used to display reserved characters (which would otherwise be interpreted as HTML code), and invisible characters (like non-breaking spaces). You can also use them in place of other characters that are difficult to type with a standard keyboard.
141	Event	CodingScripting, Glossary
141	Event	Events are assets generated by DOM elements, which can be manipulated by a Javascript code.
142	Exception	Beginner, CodingScripting, Glossary
142	Exception	An exception is a condition that interrupts normal code execution. In JavaScript syntax error are a very common source of exceptions.
143	Expando	CodingScripting, JavaScript, Reference, expando
143	Expando	Expando properties are properties added to DOM nodes with JavaScript, where those properties are not part of the object DOM specification:
144	Fallback alignment	CSS, Glossary, alignment
144	Fallback alignment	In CSS Box Alignment, a fallback alignment is specified in order to deal with cases where the requested alignment cannot be fullfilled. For example, if you specify `justify-content: space-between` there must be more than one alignment subject. If there is not, the fallback alignment is used. This is specified per alignment method, as detailed below.
145	Falsy	CodingScripting, Glossary, JavaScript
145	Falsy	A falsy (sometimes written falsey) value is a value that is considered false when encountered in a Boolean context.
146	Favicon	Glossary, Intro, favicon, user agent
146	Favicon	A favicon (favorite icon) is a tiny icon included along with a website, which is displayed in places like the browser's address bar, page tabs and bookmarks menu.
147	Fetch directive	CSP, HTTP, Security
147	Fetch directive	CSP fetch directives are used in a `Content-Security-Policy` header and control locations from which certain resource types may be loaded. For instance, `script-src` allows developers to allow trusted sources of script to execute on a page, while `font-src` controls the sources of web fonts.
148	Fetch metadata request header	Fetch Metadata Request Headers, Glossary
148	Fetch metadata request header	A fetch metadata request header is a Request header that provides additional information about the context the request originated from. These header names are prefixed with `Sec-` and thus they are Forbidden header name so headers can not be modified from JavaScript.
149	Firefox OS	B2G, Boot2Gecko, Firefox OS, Glossary, Infrastructure, Intro
149	Firefox OS	Firefox OS is Mozilla's mobile operating system, based on Linux and Mozilla Firefox powerful Gecko rendering engine. Firefox OS consists mainly of Gaia, Gecko, and Gonk.
150	firewall	DDoS, Firewall, Glossary, Security, computer network
150	firewall	A firewall is a system that filters network traffic. It can either let it pass or block it, according to some specified rules. For example, it can block incoming connections aimed at a certain port or outgoing connections to a certain IP address.
151	First contentful paint	Glossary, Performance, Reference, Web Performance
151	First contentful paint	First Contentful Paint (FCP) is when the browser renders the first bit of content from the DOM, providing the first feedback to the user that the page is actually loading. The question "Is it happening?" is "yes" when the first contentful paint completes.
152	First CPU idle	Glossary, Lighthouse, Performance, Web Performance
152	First CPU idle	First CPU Idle measures when a page is minimally interactive, or when the window is quiet enough to handle user input. It is a non-standard Google web performance metric. Generally, it occurs when most, but not necessarily all visible UI elements are interactive, and the user interface responds, on average, to most user input within 50ms. It is also known as First interactive.
153	First input delay	Glossary, Reference, Web Performance
153	First input delay	First input delay (FID) measures the time from when a user first interacts with your site (i.e. when they click a link, tap on a button, or use a custom, JavaScript-powered control) to the time when the browser is actually able to respond to that interaction.
154	First interactive	Glossary, Lighthouse, Web Performance
154	First interactive	First Interactive, also known as first CPU idle, is a non-standard web performance metric that measures when the user's window is quiet enough to handle user input, or what is termed as minimally interactive.
155	First Meaningful Paint	Glossary, Reference, Web Performance
155	First Meaningful Paint	First Meaningful Paint (FMP) is the paint after which the biggest above-the-fold layout change has happened and web fonts have loaded. It is when the answer to "Is it useful?" becomes "yes", upon first meaningful paint completion.
156	First paint	Beginner, Glossary, Performance, Web Performance
156	First paint	First Paint, part of the Paint Timing API, is the time between navigation and when the browser renders the first pixels to the screen,
157	First-class Function	CodingScripting, Glossary, JavaScript
157	First-class Function	A programming language is said to have First-class functions when functions in that language are treated like any other variable. For example, in such a language, a function can be passed as an argument to other functions, can be returned by another function and can be assigned as a value to a variable.
158	Flex Container	CSS, Glossary, flex container, flexbox
158	Flex Container	A flexbox layout is defined using the `flex` or `inline-flex` values of the `display` property on the parent item. This element then becomes a flex container, and each one of its children becomes a flex item.
159	Flex Item	CSS, Glossary, flex item, flexbox
159	Flex Item	The direct children of a Flex Container (elements with `display: flex` or `display: inline-flex` set on them) become flex items.
160	Flex	CSS, Flex, Glossary, flexbox
160	Flex	`flex` is a new value added to the CSS `display` property. Along with `inline-flex` it causes the element that it applies to to become a flex container, and the element's children to each become a flex item. The items then participate in flex layout, and all of the properties defined in the CSS Flexible Box Layout Module may be applied.
161	Flexbox	CSS, Glossary, Intro, flexbox
161	Flexbox	Flexbox is the commonly-used name for the CSS Flexible Box Layout Module, a layout model for displaying items in a single dimension — as a row or as a column.
162	Forbidden header name	Fetch, Glossary, HTTP, Headers, forbidden
162	Forbidden header name	A forbidden header name is the name of any HTTP header that cannot be modified programmatically; specifically, an HTTP request header name (in contrast with a Forbidden response header name).
163	Forbidden response header name	Glossary, HTTP, Response, forbidden
163	Forbidden response header name	A forbidden response header name is an HTTP header name (either ``Set-Cookie`` or ``Set-Cookie2``) that cannot be modified programmatically.
164	Fork	Fork, Glossary, Tools, git
164	Fork	A fork is a copy of an existing software project at some point to add someone's own modifications to the project. Basically, if the license of the original software allows, you can copy the code to develop your own version of it, with your own additions,
165	frame rate (FPS)	Animation, Glossary, Web Performance, requestAnimationFrame
165	frame rate (FPS)	A frame rate is the speed at which the browser is able to recalculate, layout and paint content to the display. The frames per second, or fps, is how many frames can be repainted in one second. The goal frame rate for in web site computer graphics is 60fps.
166	Fragmentainer	CSS, CSS Fragmentation Specification, Glossary, fragmentainer
166	Fragmentainer	A fragmentainer is defined in the CSS Fragmentation Specification as follows:
167	FTP	CodingScripting, FTP, Glossary, Protocol
167	FTP	FTP (File Transfer Protocol) was the standard protocol for many years for transferring files from one host to another over the Internet. Increasingly, though, teams and hosting accounts don't allow FTP and instead rely on a version control system like Git. You will still find it used on older hosting accounts, but it is safe to say that FTP is no longer considered best practice.
168	FTU	FTU, Firefox OS, First time use, Gaia, Glossary, Infrastructure, Intro
168	FTU	FTU (First Time Use) is the app that loads when you run a newly-installed version of Gecko on a Firefox OS device.
169	Function	CodingScripting, Glossary, IIFE, Immediately Invoked Function Expressions (IIFE), Intro, JavaScript
169	Function	A function is a code snippet that can be called by other code or by itself, or a variable that refers to the function. When a function is called, Argument are passed to the function as input, and the function can optionally return a value. A function in JavaScript is also an object.
170	Fuzz testing	Firefox, Fuzzing, Mozilla, QA, Security, Testing
170	Fuzz testing	Fuzzing is a technique for testing software using automated tools to provide invalid or unexpected input to a program or function in a program, then checking the results to see if the program crashes or otherwise acts inappropriately. This is an important way to ensure that software is stable, reliable, and secure, and we use fuzzing a lot at Mozilla.
171	Gaia	B2G, Boot2Gecko, Firefox OS, Gaia, Glossary, Infrastructure, Intro
171	Gaia	The user interface and default application suite of the Firefox OS platform.
172	Garbage collection	CodingScripting, Glossary
172	Garbage collection	Garbage collection is a term used in computer programming to describe the process of finding and deleting object which are no longer being object reference by other objects.
173	Gecko	Firefox OS, Gecko, Glossary, Infrastructure, Intro, Mozilla
173	Gecko	Gecko is the layout engine developed by the Mozilla Project and used in many apps/devices, including Mozilla Firefox and Firefox OS.
174	General header	Glossary, WebMechanics
174	General header	A general header is an Header that can be used in both request and response messages but doesn't apply to the content itself. Depending on the context they are used in, general headers are either Response header or request header (e.g. `Cache-Control`).
175	GIF	Composing, Glossary
175	GIF	GIF (Graphics Interchange Format) is an image format that uses lossless compression and can be used for animations. A GIF uses up to 8 bits per pixel and a maximum of 256 colors from the 24-bit color space.
176	GIJ	Automation, CodingScripting, Gaia, Integration, tests
176	GIJ	Gaia integration Testing. Marionette- and JavaScript- based. See GIJ.
177	Git	Collaborating, Glossary
177	Git	Git is a free, open-source, distributed Source Code Management (SCM) system. It facilitates handling code bases with distributed development teams. What sets it apart from previous SCM systems is the ability to do common operations (branching, committing, etc.) on your local development machine, without having to change the master repository or even having write access to it.
178	Global object	CodingScripting, Glossary, NeedsContent
178	Global object	A global object is an object that always exists in the global scope.
179	Global scope	CodingScripting, Glossary, NeedsContent
179	Global scope	In a programming environment, the global scope is the scope that contains, and is visible in, all other scopes.
180	Global variable	CodingScripting, Glossary
180	Global variable	A global variable is a variable that is declared in the global scope in other words, a variable that is visible from all other scopes.
181	Glyph	Glossary, SVG, typography
181	Glyph	A glyph is a term used in typography for the visual representation of one or more character.
182	Gonk	B2G, Boot2Gecko, Firefox OS, Glossary, Gonk, Infrastructure, Intro
182	Gonk	Gonk is the lower-level operating system of Firefox OS and consists of a Linux kernel (based on the Android Open Source Project (AOSP)) and userspace hardware abstraction layer (HAL).
183	Google Chrome	Browser, Chrome canary, Chrome stable, Chromium, Glossary, WebMechanics, google chrome
183	Google Chrome	Google Chrome is a free Web browser developed by Google. It's based on the Chromium open source project. Some key differences are described on the Chromium wiki. Chrome supports its own layout called Blink. Note that the iOS version of Chrome uses that platform's WebView, not Blink.
184	GPL	GPL, Glossary, License, OpenPractices, Remixing, Sharing
184	GPL	The (GNU) GPL (General Public License) is a copyleft free software license published by the Free Software Foundation. Users of a GPL-licensed program are granted the freedom to use it, read the source code, modify it and redistribute the changes they made, provided they redistribute the program (modified or unmodified) under the same license.
185	GPU	Glossary, Graphics, Infrastructure
185	GPU	The GPU (Graphics Processing Unit) is a computer component similar to the CPU (Central Processing Unit). It specializes in the drawing of graphics (both 2D and 3D) on your monitor.
186	Graceful degradation	Design, Glossary, graceful degradation
186	Graceful degradation	Graceful degradation is a design philosophy that centers around trying to build a modern web site/application that will work in the newest browsers, but falls back to an experience that while not as good still delivers essential content and functionality in older browsers.
187	Grid Areas	CSS, CSS Grids, Reference
187	Grid Areas	A grid area is one or more grid cell that make up a rectangular area on the grid. Grid areas are created when you place an item using line-based placement or when defining areas using named grid areas.
188	Grid Axis	CSS, CSS Grids
188	Grid Axis	CSS Grid Layout is a two-dimensional layout method enabling the laying out of content in rows and columns. Therefore in any grid we have two axes. The block or column axis, and the inline or row axis.
189	Grid Cell	CSS Grids
189	Grid Cell	In a CSS Grid Layout, a grid cell is the smallest unit you can have on your CSS grid. It is the space between four intersecting grid lines and conceptually much like a table cell.
190	Grid Column	CSS, CodingScripting
190	Grid Column	A grid column is a vertical track in a CSS Grid Layout, and is the space between two vertical grid lines. It is defined by the `grid-template-columns` property or in the shorthand `grid` or `grid-template` properties.
191	Grid container	CSS, Glossary, grid
191	Grid container	Using the value `grid` or `inline-grid` on an element turns it into a grid container using CSS Grid Layout, and any direct children of this element become grid items.
192	Grid Lines	CSS Grids
192	Grid Lines	Grid lines are created when you define Grid tracks in the explicit grid using CSS Grid Layout. In the following example there is a grid with three column tracks and two row tracks. This gives us 4 column lines and 3 row lines.
193	Grid Row	CSS Grids
193	Grid Row	A grid row is a horizontal track in a CSS Grid Layout, that is the space between two horizontal grid lines. It is defined by the `grid-template-rows` property or in the shorthand `grid` or `grid-template` properties.
194	Grid Tracks	CSS Grids
194	Grid Tracks	A grid track is the space between two grid lines. They are defined in the explicit grid by using the `grid-template-columns` and `grid-template-rows` properties or the shorthand `grid` or `grid-template` properties. Tracks are also created in the implicit grid by positioning a grid item outside of the tracks created in the explicit grid.
195	Grid	CSS, CSS Grid, Glossary
195	Grid	A CSS grid is defined using the `grid` value of the `display` property; you can define columns and rows on your grid using the `grid-template-rows` and `grid-template-columns` properties.
196	Guard	API, CodingScripting, Glossary, guard
196	Guard	Guard is a feature of `Headers` objects (as defined in the `Fetch_API`, which affects whether methods such as `Headers.set` and `Headers.append` can change the header's contents. For example, `immutable` guard means that headers can't be changed. For more information, read Fetch basic concepts: guard.
197	Gutters	CSS Grids
197	Gutters	Gutters or alleys are spacing between content tracks. These can be created in CSS Grid Layout using the `grid-column-gap`, `grid-row-gap`, or `grid-gap` properties.
198	Gzip compression	Glossary, compression, gzip
198	Gzip compression	Gzip is a file format used for file compression and decompression. It is based on the Deflate algorithm that allows files to be made smaller in size which allows for faster network transfers. Gzip is commonly supported by web servers and modern browsers, meaning that servers can automatically compress files with Gzip before sending them, and browsers can uncompress files upon receiving them.
199	Hash	CodingScripting, Cryptography, Glossary, Hash
199	Hash	The hash function takes a variable length message input and produces a fixed-length hash output. It is commonly in the form of a 128-bit "fingerprint" or "message digest". Hashes are very useful for cryptography — they insure the integrity of transmitted data. This provides the basis for HMAC's, which provide message authentication.
200	Head	CodingScripting, Glossary, HTML, head, metadata
200	Head	The Head is the part of an HTML document that contains metadata about that document, such as author, description, and links to CSS or JavaScript files that should be applied to the HTML.
201	High-level programming language	CodingScripting, Glossary
201	High-level programming language	A high-level programming language has a significant abstraction from the details of computer operation. It is designed to be easily understood by humans and for this reason they must be translated by another software. Unlike low-level programming languages, it may use natural language elements, or may automate (or even entirely hide) significant areas of computing systems, making the process of developing simpler and more understandable relative to a lower-level language. The amount of abstraction provided defines how "high-level" a programming language is.
202	HMAC	Cryptography, Glossary, Hash, Security
202	HMAC	HMAC is a protocol used for cryptography authenticating messages.
203	Hoisting	CodingScripting, Glossary, JavaScript
203	Hoisting	Hoisting is a term you will not find used in any normative specification prose prior to ECMAScript® 2015 Language Specification. Hoisting was thought up as a general way of thinking about how execution contexts (specifically the creation and execution phases) work in JavaScript. However, the concept can be a little confusing at first.
204	Host	Glossary, Intermediate, Web, WebMechanics
204	Host	A host is a device connected to the Internet (or a local network). Some hosts called server offer additional services like serving webpages or storing files and emails.
205	Hotlink	Glossary, WebMechanics
205	Hotlink	A hotlink (also known as an inline link) is an object (typically an image) directly linked to from another site. For example, an image hosted on site1.com is shown directly on site2.com.
206	Houdini	CSS, CSS API, Glossary, Houdini, Reference
206	Houdini	Houdini is a set of low level APIs that give developers the power to extend CSS, providing the ability to hook into the styling and layout process of a browser’s rendering engine. Houdini gives developers access to the CSS Object Model (CSSOM), enabling developers to write code the browser can parse as CSS.
207	HPKP	Glossary, Security
207	HPKP	HTTP Public Key Pinning (HPKP) is a security feature that tells a web client to associate a specific cryptographic public key with a certain web server to decrease the risk of MITM attacks with forged certificates.
208	HSTS	HTTP, Security
208	HSTS	HTTP Strict Transport Security lets a web site inform the browser that it should never load the site using HTTP and should automatically convert all attempts to access the site using HTTP to HTTPS requests instead. It consists in one HTTP header, `Strict-Transport-Security`, sent by the server with the resource.
209	HTML	CodingScripting, Glossary, HTML, l10n:priority
209	HTML	HTML (HyperText Markup Language) is a descriptive language that specifies webpage structure.
210	HTML5	CodingScripting, Glossary, HTML, HTML5, Markup
210	HTML5	The latest stable release of HTML, HTML5 takes HTML from a simple markup for structuring a document to a full app development platform. Among other features, HTML5 includes new elements and JavaScript API to enhance storage, multimedia, and hardware access.
211	HTTP/2	Glossary, HTTP, Infrastructure, Reference, Web Performance, l10n:priority
211	HTTP/2	HTTP/2 is a major revision of the HTTP network protocol
212	HTTP/3	HTTP, Intro, NeedsContent
212	HTTP/3	HTTP/3 is the upcoming major revision of the HTTP network protocol
213	HTTP header	Glossary, HTTP Header, WebMechanics
213	HTTP header	An HTTP header is a field of an HTTP request or response that passes additional context and metadata about the request or response. For example, a request message can use headers to indicate it's preferred media formats, while a response can use header to indicate the media format of the returned body. Headers are case-insensitive, begin at the start of a line and are immediately followed by a `':'` and a header-dependent value. The value finishes at the next CRLF or at the end of the message.
214	HTTP	Beginner, Glossary, HTTP, Infrastructure, Web Performance, l10n:priority
214	HTTP	The HyperText Transfer Protocol (HTTP) is the underlying network protocol that enables transfer of hypermedia documents on the World Wide Web, typically between a browser and a server so that humans can read them. The current version of the HTTP specification is called HTTP_2.
215	HTTPS	Glossary, HTTPS, Infrastructure, Security
215	HTTPS	HTTPS (*HyperText Transfer Protocol Secure*) is an encrypted version of the HTTP protocol. It uses SSL or TLS to encrypt all communication between a client and a server. This secure connection allows clients to safely exchange sensitive data with a server, such as when performing banking activities or online shopping.
216	Hyperlink	CodingScripting, Glossary, HTML, Navigation
216	Hyperlink	Hyperlinks connect webpages or data items to one another. In HTML, `a` elements define hyperlinks from a spot on a webpage (like a text string or image) to another spot on some other webpage (or even on the same page).
217	Hypertext	Glossary, Web, WebMechanics
217	Hypertext	Hypertext is text that contains links to other texts, as opposed to a single linear flow like in a novel.
218	I18N	Beginner, Credibility, Glossary, Internationalization, OpenPractices, i18n
218	I18N	i18n (from "internationalization", a 20-letter word) is the best practice that enables products or services to be readily adapted to any target culture.
219	IANA	Glossary, Infrastructure
219	IANA	IANA (Internet Assigned Numbers Authority) is a subsidiary of ICANN charged with recording and/or assigning domain name, IP address, and other names and numbers used by Internet protocol.
220	ICANN	Glossary, Infrastructure
220	ICANN	ICANN (Internet Corporation for Assigned Names and Numbers) is an international nonprofit that maintains the DNS and the record of IP address.
221	ICE	CodingScripting, Glossary, Networking, Protocols, WebRTC
221	ICE	ICE (Interactive Connectivity Establishment) is a framework used by WebRTC (among other technologies) for connecting two peers, regardless of network topology (usually for audio and video chat). This protocol lets two peers find and establish a connection with one another even though they may both be using Network Address Translator (NAT) to share a global IP address with other devices on their respective local networks.
222	IDE	CodingScripting, Glossary
222	IDE	An Integrated Development Environment (IDE) or Interactive Development environment is a software application that provides comprehensive facilities to computer programmers for software development. An IDE normally consists of a source code editor, build automation tools and a debugger.
223	Idempotent	Glossary, WebMechanics
223	Idempotent	An HTTP method is idempotent if an identical request can be made once or several times in a row with the same effect while leaving the server in the same state. In other words, an idempotent method should not have any side-effects (except for keeping statistics). Implemented correctly, the `GET`, `HEAD`, `PUT`, and `DELETE` methods are idempotent, but not the `POST` method. All safe methods are also idempotent.
224	Identifier	Beginner, CodingScripting, Glossary, Sharing
224	Identifier	An identifier is a sequence of characters in the code that identifies a variable, function, or property.
225	IDL	CodingScripting, Glossary, IDL, Interface description language
225	IDL	An IDL (Interface Description Language) is a generic language used to specified objects' interfaces apart from any specific programming language.
226	IETF	Glossary, IETF, Infrastructure, Internet
226	IETF	The Internet Engineering Task Force (IETF) is a worldwide organization that drafts specification governing the mechanisms behind the Internet, especially the TCP/IPv6 or Internet Protocol Suite.
227	IIFE	CodingScripting, DesignPattern, Functions, Glossary, JavaScript
227	IIFE	An IIFE (Immediately Invoked Function Expression) is a JavaScript function that runs as soon as it is defined.
228	IMAP	Beginner, Glossary, Infrastructure, MailNews
228	IMAP	IMAP (Internet Message Access Protocol) is a protocol used to retrieve and store emails. More recent than POP3, IMAP allows folders and rules on the server.
229	Immutable	CodingScripting, Glossary
229	Immutable	An immutable object is one whose content cannot be changed. An object can be immutable for various reasons, for example:
230	IndexedDB	API, CodingScripting, Database, Glossary, Sql
230	IndexedDB	IndexedDB is a Web API for storing large data structures within browsers and indexing them for high-performance searching. Like an SQL-based RDBMS, IndexedDB is a transactional database system. However, it uses JavaScript objects rather than fixed columns tables to store data.
231	Information architecture	Design, Glossary, information architecture
231	Information architecture	Information architecture, as applied to web design and development, is the practice of organizing the information / content / functionality of a web site so that it presents the best user experience it can, with information and services being easily usable and findable.
232	Inheritance	CodingScripting, Glossary, Inheritance, Programming Language
232	Inheritance	Inheritance is a major feature of OOP. Data abstraction can be carried up several levels, that is, class can have superclasses and subclasses.
233	Input method editor	Glossary
233	Input method editor	An input method editor (IME) is a program that provides a specialized user interface for text input. Input method editors are used in many situations:
234	Instance	Beginner, CodingScripting, Glossary, JavaScript, NeedsExample, OOP
234	Instance	An object created by a constructor is an instance of that constructor.
235	Internationalization	Glossary, Internationalization, Reference
235	Internationalization	Internationalization, often shortened to "i18n", is the adapting of a web site or web application to different languages, regional differences, and technical requirements for different regions and countries. Internationalization is the process of architecting your web application so that it can be quickly and easily adapted to various languages and regions without much engineering effort when new languages and regions are supported. Also so that a user can browse features to translate or localize the application to access all the content without breaking the layout.
236	Internet	Beginner, Glossary, Guide, Intro, NeedsContent, Tutorial, Web, WebMechanics
236	Internet	The Internet is a worldwide network of networks that uses the Internet protocol suite (also named TCP/IPv6 from its two most important protocol).
237	Intrinsic size	CSS, Glossary, Intrinsic size
237	Intrinsic size	In CSS, the intrinsic size of an element is the size it would be based on its content, if no external factors were applied to it. For example, inline elements are sized intrinsically: `width`, `height`, and vertical margin and padding have no impact, though horizontal margin and padding do.
238	IP Address	Beginner, Glossary, Infrastructure, Web
238	IP Address	An IP address is a number assigned to every device connected to a network that uses the Internet protocol.
239	IPv4	Glossary, IPv4, Infrastructure, Internet Protocol, Protocol
239	IPv4	IPv4 is the fourth version of the communication protocol underlying the Internet and the first version to be widely deployed.
240	IPv6	Glossary, IPv6, Infrastructure, Intermediate, Web, WebMechanics
240	IPv6	IPv6 is the current version of the communication protocol underlying the Internet. Slowly IPv6 is replacing IPv4, among other reasons because IPv6 allows for many different IP address.
241	IRC	Glossary, Infrastructure, Internet Relay Chat, Open Protocol, irc
241	IRC	IRC (Internet Relay Chat) is a worldwide chat system requiring an Internet connection and an IRC client, which sends and receives messages via the IRC server.
242	ISO	Glossary, ISO, Infrastructure, Web Standards, web specifications
242	ISO	ISO (International Organization for Standardization) is a global association that develops uniform criteria coordinating the companies in each major industry.
243	ISP	Glossary, ISP, Internet Service Provider, Web, WebMechanics
243	ISP	An ISP (Internet Service Provider) sells Internet access, and sometimes email, web hosting, and voice over IP, either by a dial-up connection over a phone line (formerly more common), or through a broadband connection such as a cable modem or DSL service.
244	ITU	Glossary, ITU, Standardization, organization
244	ITU	The International Telecommunication Union (ITU) is the organization authorized by the United Nations to establish standards and rules for telecommunication, including telegraph, radio, telephony and the internet.
245	Jank	Beginner, CodingScripting, Glossary, Performance, Web Performance
245	Jank	Jank refers to sluggishness in a user interface, usually caused by executing long tasks on the main thread, blocking rendering, or expending too much processor power on background processes.
246	Java	CodingScripting, Glossary, Java, Programming Language
246	Java	Java is a Compile, OOP, highly portable computer programming language.
247	JavaScript	CodingScripting, Glossary, JavaScript, l10n:priority
247	JavaScript	JavaScript (or "JS") is a programming language used most often for dynamic client-side scripts on webpages, but it is also often used on the Server-side, using a runtime such as Node.js.
248	JPEG	Beginner, Composing, Glossary, Images, JPEG
248	JPEG	JPEG (Joint Photographic Experts Group) is a commonly used method of lossy compression for digital images.
249	jQuery	Glossary, JQuery, JavaScript
249	jQuery	jQuery is a JavaScript Library that focuses on simplifying DOM manipulation, AJAX calls, and Event handling.
250	JSON	CodingScripting, Glossary, Intro, JSON, l10n:priority
250	JSON	JavaScript Object Notation (JSON) is a data-interchange format. Although not a strict subset, JSON closely resembles a subset of JavaScript syntax. Though many programming languages support JSON, JSON is especially useful for JavaScript-based apps, including websites and browser extensions.
251	Key	Cryptography, Glossary, Security
251	Key	A key is a piece of information used by a cipher for encryption and/or decryption.
252	Keyword	Glossary, Keyword, Search, keyword search
252	Keyword	A keyword is a word or phrase that describes content. Online keywords are used as queries for search engines or as words identifying content on websites.
253	Latency	Audio, Glossary, Media, Networking, Reference, Video, Web Performance, latency
253	Latency	Latency is the network time it takes for a requested resource to reach its destination. Low latency is good, meaning there is little or no delay. High latency is bad, meaning it takes a long time for the requested resource to reach its destination.
254	Layout viewport	CodingScripting, Glossary, Layout, layout viewport, viewport
254	Layout viewport	The layout viewport is the viewport into which the browser draws a web page. Essentially, it represents what is available to be seen, while the visual viewport represents what is currently visible on the user's display device.
255	Lazy load	Glossary, Lazy loading, Reference, Web Performance
255	Lazy load	Lazy loading is a strategy that delays the loading of some assets (e.g., images) until they are needed by the user based on the user's activity and navigation pattern; typically, these assets are only loaded when they are scrolled into view.
256	LGPL	Glossary, License, OpenPractices, Remixing, Sharing
256	LGPL	LGPL (GNU Lesser General Public License) is a free software license published by the Free Software Foundation. The LGPL provides a more permissive alternative for the strictly copyleft GPL. While any derivative work using a GPL-licensed program must be released under the same terms (free to use, share, study, and modify), the LGPL only requires the LGPL-licensed component of the derivative program to continue using the LGPL, not the whole program. LGPL is usually used to license shared components such as libraries (`.dll`, `.so`, `.jar`, etc.).
257	Ligature	CSS, Design, Glossary
257	Ligature	A ligature is a joining of two characters into one shape. For example, in French "œ" is a ligature of "oe".
258	Local scope	CodingScripting, Glossary, NeedsContent
258	Local scope	Local scope is a characteristic of variable that makes them local (i.e., the variable name is only bound to its value within a scope which is not the global scope).
259	Local variable	CodingScripting, Glossary, NeedsContent
259	Local variable	A variable whose name is bound to its value only within a local scope.
260	Locale	Composing, Glossary, Locale
260	Locale	Locale is a set of language- or country-based preferences for a user interface.
261	Localization	Collaborating, Glossary, Intro, Localization, WebMechanics
261	Localization	Localization (l10n) is the process of adapting a software user interface to a specific culture.
262	Long task	Glossary, Long Tasks API, Reference, Web Performance
262	Long task	A long task is a task that takes more than 50ms to complete.
263	Loop	CodingScripting, Glossary, control flow, programming
263	Loop	A loop is a sequence of instructions that is continually repeated until a certain condition is met in computer programming. An example would be the process of getting an item of data and changing it, and then making sure some conditional is checked such as, if a counter has reached a prescribed number.
264	Lossless compression	Beginner, Composing, Glossary, Images, Web Performance, compression, lossless
264	Lossless compression	Lossless compression is a class of data compression algorithms that allows the original data to be perfectly reconstructed from the compressed data. Lossless compression methods are reversible. Examples of lossless compression include GZIP, Brotli, WebP, and PNG,
265	lossy compression	Beginner, Composing, Glossary, Images, JPEG, Lossy, Web Performance, compression
265	lossy compression	Lossy compression, or irreversible compression, is a data-compression method that uses inexact approximations and partial-data discarding to represent content. In simpler terms: lossy compression causes data from the initial file to be lost, possibly causing degradation in quality. The process of such compression is irreversible; once lossy compression of the content has been performed, the content cannot be restored to its original state. Therefore, content that has undergone lossy compression should generally not be further edited.
266	LTR (Left To Right)	Composing, Glossary, Localization
266	LTR (Left To Right)	LTR (Left To Right) is a locale property indicating that text is written from left to right. For example, the `en-US` locale (for US English) specifies left-to-right.
267	Main Axis	CSS, Glossary, Main axis, flexbox
267	Main Axis	The main axis in flexbox is defined by the direction set by the `flex-direction` property. There are four possible values for `flex-direction`. These are:
268	Main thread	Glossary, Reference, Web Performance
268	Main thread	The main thread is where a browser processes user events and paints. By default, the browser uses a single thread to run all the JavaScript in your page, as well as to perform layout, reflows, and garbage collection. This means that long-running JavaScript functions can block the thread, leading to an unresponsive page and a bad user experience.
269	markup	Glossary, Intro, Markup, NeedsContent
269	markup	A markup language is one that is designed for defining and presenting text. HTML (HyperText Markup Language), is an example of a markup language.
270	MathML	CodingScripting, Glossary, MathML, Mathematical Markup Language, XML
270	MathML	MathML (an XML application) is an open standard for representing mathematical expressions in webpages. In 1998 the W3C first recommended MathML for representing mathematical expressions in the browser. MathML has other applications also including scientific content and voice synthesis.
271	Media	Disambiguation, Glossary
271	Media	The term media is an overloaded one when talking about the web; it takes on different meanings depending on the context.
272	Media (Audio-visual presentation)	Audio, Glossary, Media, Multimedia, Video
272	Media (Audio-visual presentation)	The term media (more accurately, multimedia) refers to audio, video, or combined audio-visual material such as music, recorded speech, movies, TV shows, or any other form of content that is presented over a period of time.
273	Media (CSS)	CSS, Glossary, Intro, Media
273	Media (CSS)	In the context of CSS (Cascading Style Sheets), the term media refers to the destination to which the document is to be drawn by the rendering engine.
274	Metadata	CodingScripting, Glossary, HTML, metadata
274	Metadata	Metadata is — in its very simplest definition — data that describes data. For example, an HTML document is data, but HTML can also contain metadata in its `head` element that describes the document — for example who wrote it, and its summary.
275	Method	CodingScripting, Glossary, JavaScript
275	Method	A method is a function which is a property of an object. There are two kind of methods: Instance Methods which are built-in tasks performed by an object instance, or static method which are tasks that are called directly on an object constructor.
276	Microsoft Edge	Browser, Glossary, Infrastructure
276	Microsoft Edge	Microsoft Edge is a free-of-cost graphical World Wide Web browser bundled with Windows 10 and developed by Microsoft since 2014. Initially known as Spartan, Edge replaced the longstanding browser Microsoft Internet Explorer.
277	Microsoft Internet Explorer	Browser, Glossary, Internet Explorer, Microsoft, Microsoft Internet Explorer, Navigation, Web Browser, Windows, Windows Operating System
277	Microsoft Internet Explorer	Internet Explorer (or IE) is a free graphical browser maintained by Microsoft for legacy enterprise uses. Microsoft Edge is currently the default Windows browser.
278	Middleware	CodingScripting, Glossary
278	Middleware	Middleware is a (loosly defined) term for any software or service that enables the parts of a system to communicate and manage data. It is the software that handles communication between components and input/output, so developers can focus on the specific purpose of their application.
279	MIME type	Glossary, WebMechanics
279	MIME type	A MIME type (now properly called "media type", but also sometimes "content type") is a string sent along with a file indicating the type of the file (describing the content format, for example, a sound file might be labeled `audio/ogg`, or an image file `image/png`).
280	mime	Beginner, Glossary, Infrastructure, MIME
280	mime	MIME "Multipurpose internet mail extensions" is a standard to describe documents in other forms beside ASCII text, e.g. audio, video and images. Initially used for E-Mail attachments, it has become the de facto standard to define types of documents anywhere.
281	minification	Glossary, Performance, Reference, Web Performance
281	minification	Minification is the process of removing unnecessary or redundant data without affecting how a resource is processed by the browser.
282	MitM	Glossary, Security
282	MitM	A Man-in-the-middle attack (MitM) intercepts a communication between two systems. For example, a Wi-Fi router can be compromised.
283	Mixin	CodingScripting, Glossary, Method, Mixin, Property
283	Mixin	A mixin is a class or interface in which some or all of its method and/or property are unimplemented, requiring that another class or interface provide the missing implementations.
284	Mobile First	Design, Glossary, Layout, Layout mobile
284	Mobile First	Mobile first, a form of progressive enhancement, is a web-development and web-design approach that focuses on prioritizing design and development for mobile screen sizes over design and development for desktop screen sizes. The rationale behind the mobile-first approach is to provide users with good user experiences at all screen sizes—by starting with creating a user experience that works well on small screens, and then building on top of that to further enrich the user experience as the screen size increases. The mobile-first approach contrasts with the older approach of designing for desktop screen sizes first, and then only later adding some support for small screen sizes.
285	Modem	Infrastructure, Navigation
285	Modem	A modem ("modulator-demodulator") is a device that converts digital information to analog signals and vice-versa, for sending data through networks.
286	Modern web apps	Composing, Glossary, Modern web apps
286	Modern web apps	See Progressive web apps
287	Modularity	CodingScripting, Glossary
287	Modularity	The term Modularity refers to the degree to which a system's components may be separated and recombined, it is also division of a software package into logical units. The advantage of a modular system is that one can reason the parts independently
288	Mozilla Firefox	Browser, Firefox, Glossary, Infrastructure, Mozilla, Mozilla Firefox
288	Mozilla Firefox	Mozilla Firefox is a free open-source browser whose development is overseen by the Mozilla Corporation. Firefox runs on Windows, OS X, Linux, and Android.
289	Mutable	CodingScripting, Glossary, NeedsContent
289	Mutable	Mutable is a type of variable that can be changed. In JavaScript, only Object and Array are mutable, not primitive.
290	MVC	Glossary, Infrastructure, Intro, MVC, Model View Controller
290	MVC	MVC (Model-View-Controller) is a pattern in software design commonly used to implement user interfaces, data, and controlling logic. It emphasizes a separation between the software’s business logic and display. This "separation of concerns" provides for a better division of labor and improved maintenance. Some other design patterns are based on MVC, such as MVVM (Model-View-Viewmodel), MVP (Model-View-Presenter), and MVW (Model-View-Whatever).
291	Namespace	CodingScripting, Glossary, Operating System
291	Namespace	Namespace is a context for identifiers, a logical grouping of names used in a program. Within the same context and same scope, an identifier must uniquely identify an entity.
292	NaN	CodingScripting, Computing, Glossary, NaN
292	NaN	NaN (Not a Number) is a numeric Type that means an undefined value or value that cannot be represented, especially results of floating-point calculations.
293	NAT	Beginner, Glossary, Infrastructure, WebMechanics, WebRTC
293	NAT	NAT (Network Address Translation) is a technique for letting multiple computers share an IP address. NAT assigns unique addresses to each computer on the local network and adjusts incoming/outgoing network traffic to send data to the right place.
294	Native	CodingScripting, Glossary
294	Native	A native application has been compiled to run on the hardware/software environment that comprises the targeted architecture.
295	Navigation directive	CSP, HTTP, Security
295	Navigation directive	CSP navigation directives are used in a `Content-Security-Policy` header and govern to which location a user can navigate to or submit a form to, for example.
296	Netscape Navigator	Browser, Glossary, Navigation, Netscape, Netscape Navigator
296	Netscape Navigator	Netscape Navigator or Netscape was a leading browser in the 1990s. Netscape was based on Mosaic and the Netscape team was led by Marc Andreessen, a programmer who also wrote code for Mosaic.
297	Network throttling	Glossary, RUM, Reference, Synthetic monitoring, Web Performance
297	Network throttling	Network throttling is an intentional slowing down of internet speed. In web performance, network throttling, or network condition emulation, it is used to emulate low bandwidth conditions experienced by likely a large segment of a site's target user base.
298	NNTP	Glossary, Infrastructure
298	NNTP	NNTP (Network News Transfer Protocol) is a protocol used to transfer Usenet messages from client to server or between servers.
299	Node.js	Glossary, Infrastructure, JavaScript, node.js
299	Node.js	Node.js is a cross-platform JavaScript runtime environment that allows developers to build server-side and network applications with JavaScript.
300	Node	Disambiguation, Glossary
300	Node	The term node can have several meanings depending on the context. It may refer to:
301	Node (DOM)	CodingScripting, Glossary
301	Node (DOM)	In the context of the DOM, a node is a single point in the node tree. Various things that are nodes are the document itself, elements, text, and comments.
302	Node (networking)	Glossary, Infrastructure
302	Node (networking)	In networking, a node is a connection point in the network. In physical networks, a node is usually a device, like a computer or a router.
303	non-normative	Glossary, Infrastructure, Specification, Standardization
303	non-normative	Software specification often contains information marked as non-normative or informative, which means that those are provided there for the purpose of helping the readers to understand the specification better or to show an example or a best practice, and not needed to be followed as a rule. Sections that contain official part of the specification that must be followed are often marked as normative.
304	Normative	Glossary, Infrastructure, Specification, Standardization
304	Normative	Normative is a word commonly used in software specification to denote sections that are standardized and must be followed as a rule. Specifications might also contain sections that are marked as non-normative or informative, which means those are provided there for the purpose of helping the reader understand the specifications better or to showcase an example or best practice, which need not be followed as a rule.
305	Null	CodingScripting, Glossary
305	Null	In computer science, a `null` value represents a reference that points, generally intentionally, to a nonexistent or invalid object or address. The meaning of a null reference varies among language implementations.
306	Nullish value	Glossary, JavaScript, Null, Primitive, undefined
306	Nullish value	In JavaScript, a nullish value is the value which is either `null` or `undefined`. Nullish values are always falsy.
307	Number	CodingScripting, Glossary, JavaScript
307	Number	In JavaScript, Number is a numeric data type in the double-precision 64-bit floating point format (IEEE 754). In other programming languages different numeric types exist; for example, Integers, Floats, Doubles, or Bignums.
308	Object reference	CodingScripting, Glossary
308	Object reference	A link to an object. Object references can be used exactly like the linked objects.
309	Object	CodingScripting, Glossary, Intro, Object
309	Object	Object refers to a data structure containing data and instructions for working with the data. Objects sometimes refer to real-world things, for example a `car` or `map` object in a racing game. JavaScript, Java, C++, Python, and Ruby are examples of OOP languages.
310	OOP	Beginner, CodingScripting, Glossary
310	OOP	OOP (Object-Oriented Programming) is an approach in programming in which data is encapsulated within object and the object itself is operated on, rather than its component parts.
311	OpenGL	CodingScripting, Glossary, OpenGL
311	OpenGL	OpenGL (Open Graphics Library)is a cross-language, multi-platform application programming interface (API) for rendering 2D and 3D vector graphics. The API is typically used to interact with a graphics processing unit (GPU), to achieve hardware-accelerated rendering.
312	OpenSSL	Glossary, Security
312	OpenSSL	OpenSSL is an open-source implementation of SSL and TLS.
313	Opera Browser	Browser, Glossary, Navigation, Opera, Opera Browser
313	Opera Browser	Opera is the fifth most used web browser, publicly released in 1996 and initially running on Windows only. Opera uses Blink as its layout engine since 2013 (before that, Presto). Opera also exists in mobile and tablet versions.
314	Operand	CodingScripting, Glossary
314	Operand	An operand is the part of an instruction representing the data manipulated by the operator. For example, when you add two numbers, the numbers are the operand and "+" is the operator.
315	Operator	CodingScripting, Glossary
315	Operator	Reserved syntax consisting of punctuation or alphanumeric characters that carries out built-in functionality. For example, in JavaScript the addition operator ("+") adds numbers together and concatenates strings, whereas the "not" operator ("!") negates an expression — for example making a `true` statement return `false`.
316	Origin	Glossary, Security, WebMechanics, origin
316	Origin	Web content's origin is defined by the scheme (protocol), host (domain), and port of the URL used to access it. Two objects have the same origin only when the scheme, host, and port all match.
317	OTA	Glossary, Infrastructure, Intro, OTA, Over the air, updates
317	OTA	Over The Air (OTA) refers to automatic updating of software on connected devices from a central server. All device owners receiving a given set of updates are on the same "channel", and each device often can access several channels (e.g. for production or engineering builds).
318	OWASP	Glossary, Security
318	OWASP	OWASP (Open Web Application Security Project) is a non-profit organization and worldwide network that works for security in Free Software, especially on the Web.
319	P2P	Glossary, Infrastructure, Networking, P2P
319	P2P	P2P (Peer-to-peer) is a computer networking architecture in which all participating nodes (peers) have equal privileges and share the workload. P2P differs from a client-server network architecture, where multiple client nodes connect to centralized servers for services. P2P is commonly found in Blockchain Applications.
320	PAC	CodingScripting, Glossary
320	PAC	A Proxy Auto-Configuration file (PAC file) is a file which contains a function, `FindProxyForURL()`, which is used by the browser to determine whether requests (including HTTP, HTTPS, and FTP) should go directly to the destination or if they need to be forwarded through a web proxy server.
321	Packet	Glossary, Network packet, Packet, Reference, TCP, Web Performance, payload
321	Packet	A packet, or network packet, is a formatted chunk of data sent over a network. The maincomponents of a network packet are the user data and control information. The user data is known as the payload. The control information is the information for delivering the payload. It consists of network addresses for the source and destination, sequencing information, and error detection codes and is generally found in packet headers and footer.
322	Page load time	Glossary, Timings, Web Performance, metrics, page load time
322	Page load time	Page load time is the time it takes for a page to load, measured from navigation start to the
323	Page prediction	Glossary, Security, Web Performance, page prediction
323	Page prediction	Page Prediction is a browser feature or script which, when enabled, tells the browser to download resources the user is likely to visit before the user requests the content. Page prediction improves performance by enabling almost instant loading of predicted content. However, page prediction may also download content a user does not seek.
324	Parameter	CodingScripting, Glossary, JavaScript
324	Parameter	A parameter is a named variable passed into a function. Parameter variables are used to import argument into functions.
325	Parent object	CodingScripting, Glossary, NeedsContent
325	Parent object	The object to which a given property or method belongs.
326	Parse	Browser, CSS, CodingScripting, Glossary, HTML, JavaScript, Web Performance
326	Parse	Parsing means analyzing and converting a program into an internal format that a runtime environment can actually run, for example the JavaScript engine inside browsers.
327	Parser	CodingScripting, Glossary
327	Parser	A parser is the module of a compiler or interpreter that parse a source code file.
328	Payload body	HTTP, HTTP Header
328	Payload body	The HTTP message payload body is the information ("payload") part of the data that is sent in the HTTP Message Body (if any), prior to `Transfer-Encoding` being applied. If transfer encoding is not used, the payload body and message body are the same thing!
329	PDF	Composing, Glossary, PDF, Portable Document Format
329	PDF	PDF (Portable Document Format) is a file format used to share documentation without depending on any particular software implementation, hardware platform, or operating system. PDF provides a digital image of a printed document, and keeps the same appearance when printed.
330	Perceived performance	Glossary, Perceived Performance, Reference, Web Performance
330	Perceived performance	Perceived performance is a subjective measure of how fast a website seems to a user based on load time and site responsiveness. This measure relies on human perception, not an objective metric like time to interactive.
331	Percent-encoding	Glossary, WebMechanics
331	Percent-encoding	Percent-encoding is a mechanism to encode 8-bit characters that have specific meaning in the context of URL. It is sometimes called URL encoding. The encoding consists of substitution: A '%' followed by the hexadecimal representation of the ASCII value of the replace character.
332	PHP	Beginner, CodingScripting, Glossary, Infrastructure, Intro, PHP
332	PHP	PHP (a recursive initialism for PHP: Hypertext Preprocessor) is an open-source server-side scripting language that can be embedded into HTML to build web applications and dynamic websites.
333	Pixel	Design, Glossary, Graphics
333	Pixel	A pixel is the smallest building block of a graphical display like a computer screen.
334	Placeholder names	Cryptography, Glossary, Security
334	Placeholder names	Placeholder names are commonly used in cryptography to indicate the participants in a conversation, without resorting to terminology such as "Party A," "eavesdropper," and "malicious attacker."
335	Plaintext	Cryptography, Glossary, Security
335	Plaintext	Plaintext refers to information that is being used as an input to an encryption algorithm, or to ciphertext that has been decrypted.
336	PNG	Beginner, Composing, Glossary, Infrastructure, PNG
336	PNG	PNG (Portable Network Graphics) is a graphics file format that supports lossless data compression.
337	Polyfill	Browser Support, CodingScripting, Glossary, JavaScript, polyfill
337	Polyfill	A polyfill is a piece of code (usually JavaScript on the Web) used to provide modern functionality on older browsers that do not natively support it.
338	Polymorphism	CodingScripting, Glossary
338	Polymorphism	Polymorphism is the presentation of one interface for multiple data types. For example, integers, floats, and doubles are implicitly polymorphic: regardless of their different types, they can all be added, subtracted, multiplied, and so on.
339	POP3	Beginner, Glossary, Infrastructure
339	POP3	POP3 (Post Office Protocol) is a very common protocol for getting emails from a mail server over a TCP connection. POP3 does not support folders, unlike the more recent IMAP4, which is harder to implement because of its more complex structure.
340	Port	Glossary, Intro, Security, computer network, port
340	Port	For a computer connected to a network with an IP address, a port is a communication endpoint. Ports are designated by numbers, and below 1024 each port is associated by default with a specific protocol.
341	Prefetch	Glossary, Prefetch, Reference, Web Performance
341	Prefetch	Prefetching is when content is downloaded in the background, this is based on the assumption that the content will likely be requested, enabling the content to load instantly if and when the user requests it. The content is downloaded and cached for anticipated future use without the user making an explicit request for it.
342	Preflight request	CORS, Glossary, HTTP, Preflight, request
342	Preflight request	A CORS preflight request is a CORS request that checks to see if the CORS protocol is understood and a server is aware using specific methods and headers.
343	Prerender	Glossary, Prefetch, Security, Web Performance, prerender
343	Prerender	With prerendering, the content is prefetched and then rendered in the background by the browser as if the content had been rendered into an invisible separate tab. When the user navigates to the prerendered content, the current content is replaced by the prerendered content instantly.
344	Presto	Glossary, Infrastructure
344	Presto	Presto was the proprietary browser layout engine used to power the Opera browser until version 15. Since then, the Opera browser is based on Chromium, which uses the Blink layout engine.
345	Primitive	CodingScripting, Glossary, JavaScript
345	Primitive	In JavaScript, a primitive (primitive value, primitive data type) is data that is not an object and has no method. There are 6 primitive data types: string, number, bigint, boolean, undefined, and symbol. There also is null, which is seemingly primitive, but indeed is a special case for every `Object`: and any structured type is derived from `null` by the Prototype Chain.
346	privileged code	privileged
346	privileged code	Privileged code - Javascript code of your extension. For example, code in content scripts.
347	Privileged	Glossary, Security
347	Privileged	Users are said to be privileged when they are granted additional rights to a system, or given ultimate access to content in a higher priority level when compared to normal users.
348	Progressive Enhancement	Accessibility, Design, Glossary
348	Progressive Enhancement	Progressive enhancement is a design philosophy that provides a baseline of essential content and functionality to as many users as possible, while delivering the best possible experience only to users of the most modern browsers that can run all the required code.
349	Progressive web apps	Composing, Glossary, Progressive web apps
349	Progressive web apps	Progressive web apps is a term used to describe the modern state of web app development. This involves taking standard web sites/apps that enjoy all the best parts of the Web — such as discoverability via search engines, being linkable via URLs, and working across multiple form factors — and supercharging them with modern APIs (such as Service Workers and Push) and features that confer other benefits more commonly attributed to native apps.
350	Promise	Glossary, Promise, Promises, asynchronous
350	Promise	A `Promise` is an object that's returned by a function that has not yet completed its work. The promise literally represents a promise made by the function that it will eventually return a result through the promise object.
351	Property	Disambiguation, Glossary
351	Property	The term property can have several meanings depending on the context. It may refer to:
352	Property (CSS)	CodingScripting, Glossary
352	Property (CSS)	A CSS property is a characteristic (like color) whose associated value defines one aspect of how the browser should display the element.
353	Property (JavaScript)	CodingScripting, Glossary
353	Property (JavaScript)	A JavaScript property is a characteristic of an object, often describing attributes associated with a data structure.
354	Protocol	Glossary, Infrastructure, Protocols
354	Protocol	A protocol is a system of rules that define how data is exchanged within or between computers. Communications between devices require that the devices agree on the format of the data that is being exchanged. The set of rules that defines a format is called a protocol.
355	Prototype-based programming	CodingScripting, Glossary
355	Prototype-based programming	Prototype-based programming is a style of OOP in which Class are not explicitly defined, but rather derived by adding properties and methods to an instance of another class or, less frequently, adding them to an empty object. In simple words: this type of style allows the creation of an Object without first defining its Class.
356	Prototype	Apps, Composing, Glossary
356	Prototype	A prototype is a model that displays the appearance and behavior of an application or product early in the development lifecycle.
357	Proxy server	Glossary, Proxy, Server
357	Proxy server	A proxy server is an intermediate program or computer used when navigating through different networks of the Internet. They facilitate access to content on the World Wide Web. A proxy intercepts requests and serves back responses; it may forward the requests, or not (for example in the case of a cache), and it may modify it (for example changing its headers, at the boundary between two networks).
358	Pseudo-class	CSS, CodingScripting, Glossary, Selector
358	Pseudo-class	In CSS, a pseudo-class selector targets elements depending on their state rather than on information from the document tree. For example, the selector `a:visited` applies styles only to links that the user has already followed.
359	Pseudo-element	CSS, CodingScripting, Glossary
359	Pseudo-element	In CSS, a pseudo-element selector applies styles to parts of your document content in scenarios where there isn't a specific HTML element to select. For example, rather than putting the first letter of each paragraph in its own element, you can style them all with `p::first-letter`.
360	Pseudocode	CodingScripting, Glossary, Pseudocode
360	Pseudocode	Pseudocode refers to code-like syntax that is generally used to indicate to humans how some code syntax works, or illustrate the design of an item of code architecture. It won't work if you try to run it as code.
361	Public-key cryptography	Cryptography, Glossary, Public-key cryptography, Security
361	Public-key cryptography	Public-key cryptography — or asymmetric cryptography — is a cryptographic system in which keys come in pairs. The transformation performed by one of the keys can only be undone with the other key. One key (the private key) is kept secret while the other is made public.
362	Python	CodingScripting, Glossary, Language, Python, programming
362	Python	Python is a high level general-purpose programming language. It uses a multi-paradigm approach, meaning it supports procedural, object-oriented, and some functional programming constructs.
363	Quality values	Glossary, WebMechanics
363	Quality values	Quality values, or q-values and q-factors, are used to describe the order of priority of values in a comma-separated list. It is a special syntax allowed in some HTTP headers and in HTML.
364	Quaternion	Definition, Glossary, Orientation, Quaternion, WebXR, rotation
364	Quaternion	A quaternion is the quotient of two 3D vectors and is used in 3D graphics and in accelerometer-based sensors to represent orientation or rotational data.
365	QUIC	Glossary, HTTP, QUIC, Reference, Web Performance
365	QUIC	Quick UDP Internet Connection, or QUIC, is an experimental multiplexed transport protocol implemented on UDP. It was developed by Google as a way to experiment with ways to improve TCP and web application delivery.
366	RAIL	Glossary, RAIL, Timings, Web Performance
366	RAIL	RAIL, an acronym for Response, Animation, Idle, and Load, is a performance model originated by the Google Chrome team in 2015, focused on user experience and performance within the browser. The performance mantra of RAIL is "Focus on the user; the end goal isn't to make your site perform fast on any specific device, it's to make users happy." There are 4 stages of interaction: page load, idle, response to input, and scrolling and animation. In acronym order, the main tenets are:
367	Raster image	CUR, Documents, Glossary, ICO, JPEG, PNG, gif, raster image
367	Raster image	A raster image is an image file defined as a grid of pixels. They’re also referred to as bitmaps. Common raster image formats on the Web are JPEG, PNG, GIF, and ICO.
368	RDF	CodingScripting, Glossary, Infrastructure, OpenPractices, WebMechanics
368	RDF	RDF (Resource Description Framework) is a language developed by W3C for representing information on the World Wide Web, such as Webpages. RDF provides a standard way of encoding resource information so that it can be exchanged in a fully automated way between applications.
369	Real User Monitoring (RUM)	Glossary, RUM, Reference, Web Performance
369	Real User Monitoring (RUM)	Real User Monitoring or RUM measures the performance of a page from real users' machines. Generally, a third party script injects a script on each page to measure and report page load data for every request made. This technique monitors an application’s actual user interactions. In RUM, the third party script collects performance metrics from the real users' browsers. RUM helps identify how an application is being used, including the geographic distribution of users and the impact of that distribution on the end user experience.
370	Recursion	CodingScripting, Glossary
370	Recursion	The act of a function calling itself, recursion is used to solve problems that contain smaller sub-problems. A recursive function can receive two inputs: a base case (ends recursion) or a recursive case (resumes recursion).
371	Reference	CodingScripting, Glossary, NeedsContent
371	Reference	In the context of object, this is an object reference. On MDN, we could be talking about the JavaScript reference itself.
372	Reflow	Glossary, WebMechanics
372	Reflow	Reflow happens when a browser must process and draw part or all of a webpage again, such as after an update on an interactive site.
373	Regular expression	CodingScripting, Glossary, Regular Expression
373	Regular expression	Regular expressions (or regex) are rules that govern which sequences of characters come up in a search.
374	Rendering engine	Glossary, Infrastructure, Rendering engine, Web browser engine
374	Rendering engine	A rendering engine is software that draws text and images on the screen. The engine draws structured text from a document (often HTML), and formats it properly based on the given style declarations (often given in CSS). Examples of layout engines: Blink, Gecko, EdgeHTML, WebKit.
375	Repo	Glossary, Infrastructure, Intro, Repo, Repository
375	Repo	In a revision control system like Git or SVN, a repo (i.e. "repository") is a place that hosts an application's code source, together with various metadata.
376	Reporting directive	CSP, HTTP, Policy, Reporting, Security, Violation
376	Reporting directive	CSP reporting directives are used in a `Content-Security-Policy` header and control the reporting process of CSP violations.
377	Request header	Glossary, WebMechanics
377	Request header	A request header is an header that can be used in an HTTP request to provide information about the request context, so that the server can tailor the response. For example, the `Accept` headers indicate the allowed and preferred formats of the response. Other headers can be used to supply authentication credentials (e.g. `Authorization`, to control caching, or to get information about the user agent or referrer, etc.
378	Resource Timing	Glossary, Timings, Web Performance
378	Resource Timing	Diagnosing performance issues requires performance data at the granularity of the resource. The Resource Timing API is a JavaScript API that is able to capture timing information for each individual resource that is fetched when a page is loaded.
379	Response header	Glossary, WebMechanics
379	Response header	A response header is an header that can be used in an HTTP response and that doesn't relate to the content of the message. Response headers, like `Age`, `Location` or `Server` are used to give a more detailed context of the response.
380	Responsive web design	Accessibility, Design, Glossary, Responsive web design
380	Responsive web design	Responsive Web Design (RWD) is a Web development concept focusing on making sites look and behave optimally on all personal computing devices, from desktop to mobile.
381	REST	Architecture, Beginner, Glossary, HTTP, Rest, WebMechanics
381	REST	REST(Representational State Transfer) refers to a group of software architecture design constraints that bring about efficient, reliable and scalable distributed systems.
382	RGB	Beginner, CSS, Design, Guide
382	RGB	Red Green Blue (RGB) is a color model that represents colors as mixtures of three underlying components (or channels), namely, red, green, and blue. Each color is described by a sequence of three numbers (typically between 0.0 and 1.0, or between 0 and 255) that represent the different intensities (or contributions) of red, green, and blue, in determining the final color.
383	RIL	B2G, Firefox OS, Glossary, Infrastructure, Intro, Mobile, Telephony
383	RIL	RIL (Radio Interface Layer) is a mobile operating system component which communicates between the device's software and the device's phone, radio, or modem hardware.
384	Random Number Generator	CodingScripting, Glossary
384	Random Number Generator	A PRNG (pseudorandom number generator) is an algorithm that outputs numbers in a complex, seemingly unpredictable pattern. Truly random numbers (say, from a radioactive source) are utterly unpredictable, whereas all algorithms are predictable, and a PRNG returns the same numbers when passed the same starting parameters or seed.
385	Robots.txt	Glossary, Infrastructure
385	Robots.txt	Robots.txt is a file which is usually placed in the root of any website. It decides whether crawler are permitted or forbidden access to the web site.
386	Round Trip Time (RTT)	Beginner, Glossary, Performance, Resource, Round Trip Time, Web Performance
386	Round Trip Time (RTT)	Round Trip Time (RTT) is the length time it takes for a data packet to be sent to a destination plus the time it takes for an acknowledgment of that packet to be received back at the origin. The RTT between a network and server can be determined by using the `ping` command.
387	Routers	Intro
387	Routers	There are two definitions for routers on the web:
388	RSS	Glossary, OpenPractices, RSS, Sharing, WebMechanics
388	RSS	RSS (Really Simple Syndication) refers to several XML document formats designed for publishing site updates. When you subscribe to a website's RSS feed, the website sends information and updates to your RSS reader in an RSS document called a feed, so you don't need to check all your favorite websites manually.
389	Rsync
389	Rsync	Rsync is an open-source file synchronizing tool that provides incremental file transfer It can be used over insecure and secure transports (like SSH). It is available on most Unix-based systems (such as macOS and Linux) and Windows. There are also GUI-based tools that use rsync, for example, Acrosync.
390	RTCP (RTP Control Protocol)	Glossary, Media, Networking, Protocol, RTCP, control
390	RTCP (RTP Control Protocol)	The RTP Control Protocol (RTCP) is a partner to the RTP protocol. RTCP is used to provide control and statistical information about an RTP media streaming session.
391	RTF	Composing, Format, Glossary, RTF, Rich Text Format
391	RTF	RTF (Rich Text Format) is a plain-text-based file format with support for formatting instructions (like bold or italic).
392	RTL (Right to Left)	Composing, Glossary, Localization
392	RTL (Right to Left)	RTL (Right To Left) is a locale property indicating that text is written from right to left.
393	RTP (Real-time Transport Protocol) and SRTP (Secure RTP)	Glossary, Network, Protocol, RTP
393	RTP (Real-time Transport Protocol) and SRTP (Secure RTP)	The Real-time Transport Protocol (RTP) is a network protocol which described how to transmit various media (audio, video) from one endpoint to another in a real-time fashion. RTP is suitable for video-streaming application, telephony over IP like Skype and conference technologies.
394	RTSP: Real-time streaming protocol	Glossary, Real-time streaming protocol, Reference, rtsp
394	RTSP: Real-time streaming protocol	Real-time streaming protocol (RTSP) is a network protocol that controls how the streaming of a media should occur between a server and a client. Basically, RTSP is the protocol that describes what happens when you click "Pause"/"Play" when streaming a video. If your computer were a remote control and the streaming server a television, RTSP would describe how the instruction of the remote control affects the TV.
395	Ruby	CodingScripting, Glossary, Ruby
395	Ruby	Ruby is an open-source programming language. In a world wide web context, Ruby is often used server-side with the Ruby On Rails (ROR) framework to produce websites/apps.
396	Safe	Glossary, WebMechanics
396	Safe	An HTTP method is safe if it doesn't alter the state of the server. In other words, a method is safe if it leads to a read-only operation. Several common HTTP methods are safe: `GET`, `HEAD`, or `OPTIONS`. All safe methods are also idempotent, but not all idempotent methods are safe. For example, `PUT` and `DELETE` are both idempotent but unsafe.
397	Same-origin policy	Glossary, Same-origin policy, origin
397	Same-origin policy	The same-origin policy is a critical security mechanism that restricts how a document or script loaded from one origin can interact with a resource from another origin.
398	SCM	CodingScripting, Glossary, SCM
398	SCM	SCM (Source Control Management) is a system for managing source code. Usually it refers to the use of software to handle versioning of source files. A programmer can modify source code files without being afraid of editing out useful stuff, because a SCM keeps track of how the source code has changed and who made the changes.
399	Scope	CodingScripting, Glossary, JavaScript
399	Scope	The current context of execution. The context in which value and expressions are "visible" or can be referenced. If a variable or other expression is not "in the current scope," then it is unavailable for use. Scopes can also be layered in a hierarchy, so that child scopes have access to parent scopes, but not vice versa.
400	Screen reader	Accessibility, Glossary, Screen reader, Voice Over, Voiceover
400	Screen reader	Screen readers are software applications that attempt to convey what is seen on a screen display in a non-visual way, usually as text to speech, but also into braille or sound icons. Screen readers are essential to people who are blind, as well as useful to people who are visually impaired, illiterate, or have a learning disability. There are some browser extension screen readers, but most screen readers operate system-wide for all user applications, not just the browser.
401	Script-supporting element	Glossary, HTML, HTML Content Categories, scripts
401	Script-supporting element	In an HTML document, script-supporting elements are those elements that don't directly contribute to the appearance or layout of the page; instead, they're either scripts or contain information that's only used by scripts.
402	Scroll container	Glossary, scroll container
402	Scroll container	A scroll container is created by applying `overflow: scroll` to a container, or `overflow: auto` when there is enough content to cause overflow.
403	Scrollport	Glossary, scrollport
403	Scrollport	The scrollport is the visual viewport of a Scroll container in a document. A scroll container is created by applying `overflow: scroll` to a container, or `overflow: auto` when there is enough content to cause overflow. The scrollport coincides with the padding box of that container and represents the content that can be seen as the box is scrolled.
404	SCTP	Glossary, Infrastructure, SCTP, WebRTC
404	SCTP	SCTP (Stream Control Transmission Protocol) is an IETF standard for a transport protocol which enables the reliable, in-order transmission of messages while offering congestion control, multi-homing, and other features to improve reliability and stability of the connection. It's used for sending traditional telephone calls over the Internet, but is also used for WebRTC data.
405	SDP	Advanced, Collaborating, Glossary, Infrastructure, Protocol, WebRTC
405	SDP	SDP (Session Description Protocol) is the standard describing a P2P connection. SDP contains the codec, source address, and timing information of audio and video.
406	Search engine	Definition, Glossary, Indexing, Search Engine, Searching, Web Crawling, WebMechanics, World Wide Web, details, google
406	Search engine	A search engine is a software system that collects information from the World Wide Web and presents it to users who are looking for specific information.
407	Second-level Domain	Glossary, Infrastructure
407	Second-level Domain	No summary!
408	Self-Executing Anonymous Function	Glossary
408	Self-Executing Anonymous Function	A JavaScript function that runs as soon as it is defined. Also known as an IIFE (Immediately Invoked Function Expression).
409	Semantics	CodingScripting, Glossary, HTML, semantics
409	Semantics	In programming, Semantics refers to the meaning of a piece of code — for example "what effect does running that line of JavaScript have?", or "what purpose or role does that HTML element have" (rather than "what does it look like?".)
410	SEO	Glossary, Intro, SEO, Search, WebMechanic
410	SEO	SEO (Search Engine Optimization) is the process of making a website more visible in search results, also termed improving search rankings.
411	Serialization	CodingScripting, Glossary, JavaScript, Serialization
411	Serialization	The process whereby an object or data structure is translated into a format suitable for transferral over a network, or storage (e.g. in an array buffer or file format).
412	Server Timing	Glossary, Reference, Server Timing, Web Performance
412	Server Timing	The Server Timing specification enables the server to communicate performance metrics from the request-response cycle to the user agent, and utilizes a JavaScript interface to allow applications to collect, process, and act on these metrics to optimize application delivery.
413	Server	Glossary, Infrastructure, Networking, Protocol, Server
413	Server	A server is a software or hardware offering a service to a user, usually referred to as client. A hardware server is a shared computer on a network, usually powerful and housed in a data center. A software server (often running on a hardware server) is a program that provides services to client programs or a UI to human clients.
414	Session Hijacking	Glossary, Security, session hijacking
414	Session Hijacking	Session hijacking occurs when an attacker takes over a valid session between two computers. The attacker steals a valid session ID in order to break into the system and snoop data.
415	SGML	CodingScripting, Composing, Glossary, SGML
415	SGML	The Standard Generalized Markup Language (SGML) is an ISO specification for defining declarative markup languages.
416	Shadow tree	DOM, Glossary, Shadow Tree, shadow dom
416	Shadow tree	A shadow tree is a tree of DOM node whose topmost node is a shadow root; that is, the topmost node within a shadow DOM. A shadow tree is a hidden set of standard DOM nodes which is attached to a standard DOM node that serves as a host. The hidden nodes are not directly visible using regular DOM functionality, but require the use of a special Shadow DOM API to access.
417	Shim	CodingScripting, Glossary
417	Shim	A shim is a piece of code used to correct the behavior of code that already exists, usually by adding new API that works around the problem. This differs from a polyfill, which implements a new API that is not supported by the stock browser as shipped.
418	Signature	Disambiguation, Glossary
418	Signature	The term signature can have several meanings depending on the context. It may refer to:
419	Signature (functions)	CodingScripting, Glossary, Java, JavaScript
419	Signature (functions)	A function signature (or type signature, or method signature) defines input and output of Function or Method.
420	Signature (security)	Cryptography, Glossary, Privacy, Security
420	Signature (security)	A signature, or digital signature, is a protocol showing that a message is authentic.
421	SIMD	CodingScripting, Glossary, JavaScript
421	SIMD	SIMD (pronounced "sim-dee") is short for Single Instruction/Multiple Data which is one classification of computer architectures. SIMD allows one same operation to be performed on multiple data points resulting in data level parallelism and thus performance gains — for example, for 3D graphics and video processing, physics simulations or cryptography, and other domains.
422	Simple header	CORS, Glossary, HTTP, Infrastructure
422	Simple header	Old term for CORS-safelisted request header.
423	Simple response header	CORS, Glossary, HTTP
423	Simple response header	Old term for CORS-safelisted response header.
424	SISD	CodingScripting, Glossary
424	SISD	SISD is short for Single Instruction/Single Data which is one classification of computer architectures. In SISD architecture, a single processor executes a single instruction and operates on a single data point in memory.
425	Site map	Accessibility, Glossary, Search, Site map
425	Site map	A site map or sitemap is a list of pages of a web site.
426	Site	Glossary, Security, WebMechanics
426	Site	The site of a piece of web content is determined by the registrable domain of the host within the origin. This is computed by consulting a Public Suffix List to find the portion of the host which is counted as the public suffix (e.g. `com`, `org` or `co.uk`).
427	SLD	Glossary, Infrastructure
427	SLD	An SLD (Second Level Domain) is the domain name that is located right before a TLD.
428	Sloppy mode	CodingScripting, Glossary, JavaScript, Sloppy
428	Sloppy mode	ECMAScript 5 and later let scripts opt in to a new strict mode, which alters the semantics of JavaScript in several ways to improve its resiliency and which make it easier to understand what's going on when there are problems.
429	Slug	Community, Glossary, Intermediate, MDN, URL, Web
429	Slug	A Slug is the unique identifying part of a web address, typically at the end of the URL. In the context of MDN, it is the portion of the URL following "<locale>/docs/".
430	Smoke Test	Composing, Glossary, Intro, QA, Testing
430	Smoke Test	A smoke test consists of functional or unit tests of critical software functionality. Smoke testing comes before further, in-depth testing.
431	SMPTE (Society of Motion Picture and Television Engineers)	Engineers, Glossary, Motion Picture, Movies, SMPTE, Specifications, Television, standards
431	SMPTE (Society of Motion Picture and Television Engineers)	The Society of Motion Picture and Television Engineers (SMPTE) is the professional association of engineers and scientists that develop and define standards and technologies used to create, broadcast, store, and present entertainment media.
432	SMTP	Beginner, Collaboration, Glossary, Infrastructure, Sharing
432	SMTP	SMTP (Simple Mail Transfer Protocol) is a protocol used to send a new email. Like POP3 and NNTP, it is a state machine-driven protocol.
433	Snap positions	Glossary, snap positions
433	Snap positions	A scroll container may set snap positions — points that the scrollport will stop moving at after a scrolling operation is completed. This allows a scrolling experience that gives the effect of paging through content rather than needing to drag content into view.
434	SOAP	Glossary, Infrastructure, SOAP, WebMechanics
434	SOAP	SOAP (Simple Object Access Protocol) is a protocol for transmitting data in XML format. Mozilla Firefox removed support for SOAP in 2008.
435	SPA (Single-page application)	Glossary, SPA, single-page app
435	SPA (Single-page application)	An SPA (Single-page application) is a web app implementation that loads only a single web document, and then updates the body content of that single document via JavaScript APIs such as `XMLHttpRequest` and Fetch when different content is to be shown.
436	Specification	Glossary, OpenPractices, Standardization
436	Specification	A specification is a document that lays out in detail what functionality or attributes a product must include before delivery. In the context of describing the Web, the term "specification" (often shortened to "spec") generally means a document describing a language, technology, or API which makes up the complete set of open Web technologies.
437	Speculative parsing	Advanced, HTML, HTML5, NeedsUpdate, Web Development, Web Performance
437	Speculative parsing	Traditionally in browsers the HTML parser ran on the main thread and was blocked after a `</script>` tag until the script has been retrieved from the network and executed. Some HTML parser, such as Firefox since Firefox 4, support speculative parsing off of the main thread. It parses ahead while scripts are being downloaded and executed. The HTML parser starts speculative loads for scripts, style sheets and images it finds ahead in the stream and runs the HTML tree construction algorithm speculatively. The upside is that when a speculation succeeds, there's no need to reparse the part of the incoming file that was already scanned for scripts, style sheets and images. The downside is that there's more work lost when the speculation fails.
438	Speed index	Glossary, Performance, Reference, Web Performance
438	Speed index	Speed Index (SI) is a page load performance metric that shows you how quickly the contents of a page are visibly populated. It is the average time at which visible parts of the page are displayed. Expressed in milliseconds, and dependent on the size of the viewport, the lower the score, the better.
439	SQL Injection	Glossary, Security, Sql, Sql Injection, Webapp
439	SQL Injection	SQL injection takes advantage of Web apps that fail to validate user input. Hackers can maliciously pass SQL commands through the Web app for execution by a backend database.
440	SQL	CodingScripting, Database, Glossary, Sql
440	SQL	SQL (Structured Query Language) is a descriptive computer language designed for updating, retrieving, and calculating data in table-based databases.
441	SRI	CSP, Security
441	SRI	Subresource Integrity (SRI) is a security feature that enables browsers to verify that files they fetch (for example, from a CDN) are delivered without unexpected manipulation. It works by allowing you to provide a cryptographic hash that a fetched file must match.
442	Secure Sockets Layer (SSL)	Glossary, SSL, Security, TLS, Web Performance
442	Secure Sockets Layer (SSL)	Secure Sockets Layer, or SSL, was the old standard security technology for creating an encrypted network link between a server and client, ensuring all data passed is private and secure. The current version of SSL is version 3.0, released by Netscape in 1996, and has been superseded by the TLS protocol.
443	Stacking context	CSS, CodingScripting, Glossary
443	Stacking context	Stacking context refers to how elements on a webpage appear to sit on top of other elements, just as you can arrange index cards on your desk to lie side-by-side or overlap each other.
444	State machine	CodingScripting, Finite, Glossary, Input, Mealy, Moore, State Machine, Turing Machine
444	State machine	A state machine is a mathematical abstraction used to design algorithms. A state machine reads a set of inputs and changes to a different state based on those inputs.
445	Statement	Beginner, CodingScripting, Glossary
445	Statement	In a computer programming language, a statement is a line of code commanding a task. Every program consists of a sequence of statements.
446	Static method	CodingScripting, Glossary, JavaScript, Method, Static, Static Method
446	Static method	A static method (or static function) is a method defined as a member of an object but is accessible directly from an API object's constructor, rather than from an object instance created via the constructor.
447	Static typing	CodingScripting, Glossary, Type
447	Static typing	A statically-typed language is a language (such as Java, C, or C++) where variable types are known at compile time. In most of these languages, types must be expressly indicated by the programmer; in other cases (such as OCaml), type inference allows the programmer to not indicate their variable types.
448	Strict mode	Glossary, JavaScript, Reference
448	Strict mode	JavaScript's strict mode is a way to opt in to a restricted variant of JavaScript, thereby implicitly opting-out of "Sloppy_mode". Strict mode isn't just a subset: it intentionally has different semantics from normal code.
449	String	Beginner, CodingScripting, Glossary, String
449	String	In any computer programming language, a string is a sequence of character used to represent text.
450	STUN	Glossary, Infrastructure, STUN, WebMechanics, WebRTC
450	STUN	STUN (Session Traversal Utilities for NAT) is an auxiliary protocol for transmitting data around a NAT (Network Address Translator). STUN returns the IP address, port, and connectivity status of a networked computer behind a NAT.
451	Style origin	CSS, Glossary, Style, Style Origin, origin
451	Style origin	In CSS, there are three categories of sources for style changes. These categories are called style origins. They are the user agent origin, user origin, and the author origin.
452	Stylesheet	Glossary, StyleSheet
452	Stylesheet	A stylesheet is a set of CSS rules used to control the layout and design of a webpage or document. Internal stylesheets are placed inside a `style` element inside the `head` of a web document, and external stylesheets are placed inside a separate `.css` file, which is applied to a document by referencing the file inside a `link` element in the document's head.
453	SVG	Beginner, CodingScripting, Glossary, Graphics, SVG, l10n:priority
453	SVG	Scalable Vector Graphics (SVG) is a 2D vector image format based on an XML syntax.
454	SVN	Collaborating, Glossary
454	SVN	Apache Subversion (SVN) is a free source control management (SCM) system. It allows developers to keep a history of text and code modifications. Although SVN can also handle binary files, we do not recommend that you use it for such files.
455	Symbol	Data Type, ECMAScript 2015, Glossary, JavaScript, Sharing, Symbol
455	Symbol	In JavaScript, Symbol is a Primitive.
456	Symmetric-key cryptography	Cryptography, Glossary, Security, Symmetric-key cryptography
456	Symmetric-key cryptography	Symmetric-key cryptography is a term used for cryptographic algorithms that use the same key for encryption and for decryption. The key is usually called a "symmetric key" or a "secret key".
457	Synchronous	Glossary, Web, WebMechanics
457	Synchronous	Synchronous refers to real-time communication where each party receives (and if necessary, processes and replies to) messages instantly (or as near to instantly as possible).
458	Syntax error	CodingScripting, Glossary, JavaScript
458	Syntax error	An exception caused by the incorrect use of a pre-defined syntax. Syntax errors are detected while compiling or parsing source code.
459	Syntax	CodingScripting, Glossary, Syntax
459	Syntax	Syntax specifies the required combination and sequence of character making up correctly structured code. Syntax generally includes grammar and the rules that apply to writing it, such as indentation requirements in Python.
460	Synthetic monitoring	Glossary, RUM, Reference, Synthetic monitoring, Web Performance
460	Synthetic monitoring	Synthetic monitoring involves monitoring the performance of a page in a 'laboratory' environment, typically with automation tooling in an environment that is as consistent as possible.
461	Table Grid Box	CSS, Glossary, Tables
461	Table Grid Box	The Table Grid Box is a block level box which contains all of the table internal boxes, excluding the caption. The box which includes the caption is referred to as the Table Wrapper Box.
462	Table Wrapper Box	CSS, Glossary, Tables
462	Table Wrapper Box	The Table Wrapper Box is the box generated around table grid boxes which accounts for the space needed for any caption displayed for the table. This box will become the containing block for absolutely positioned items where the table is the containing block.
463	Tag	CodingScripting, Glossary, HTML, Intro
463	Tag	In HTML, a tag is used for creating an element.
464	TCP handshake	Glossary, Networking, SSL, Security, TCP, TCP handshake, TLS, Web Performance
464	TCP handshake	TCP uses a three-way handshake (aka TCP-handshake, three message handshake, and/or SYN-SYN-ACK) to set up a TCP/IP connection over an IP based network.
465	TCP slow start	Glossary, Infrastructure, Networking, TCP, Transmission Control Protocol, Web Performance, data
465	TCP slow start	TCP slow start helps buildup transmission speeds to the network's capabilities. It does this without initially knowing what those capabilities are and without creating congestion. TCP slow start is an algorithm used to detect the available bandwidth for packet transmission, and balances the speed of a network connection. It prevents the appearance of network congestion whose capabilities are initially unknown, and slowly increases the volume of information diffused until the network's maximum capacity is found.
466	TCP	Glossary, Infrastructure, Networking, TCP, Transmission Control Protocol, data
466	TCP	TCP (Transmission Control Protocol) is an important network protocol that lets two hosts connect and exchange data streams. TCP guarantees the delivery of data and packets in the same order as they were sent. Vint Cerf and Bob Kahn, who were DARPA scientists at the time, designed TCP in the 1970s.
467	Telnet	Glossary, Infrastructure
467	Telnet	Telnet is a command line tool and an underlying TCP/IP protocol for accessing remote computers.
468	Texel	3D, Drawing, Glossary, Graphics, Texel, Texture
468	Texel	A Texel is a single-pixel within a texture map, which is an image that gets used (in whole or in part) as the image presented on a polygon's surface within a 3D rendered image. It is not to be confused with pixel which is the unit of screen space. This is one of the main differences between Texel’s and pixels, pixels are image data. Texel components are made up of subjective data, therefore they can be an image as well as a depth map.
469	Thread	Glossary, Thread, asynchronous
469	Thread	Thread in computer science is the execution of running multiple tasks or programs at the same time. Each unit capable of executing code is called a thread.
470	Three js	Browser, CodingScripting, JavaScript, Programming Language, three.js
470	Three js	three.js is a JavaScript-based WebGL engine that can run GPU-powered games and other graphics-powered apps straight from the browser. The three.js library provides many features and API for drawing 3D scenes in your browser.
471	Time to first byte	Glossary, Performance, Reference, Web Performance
471	Time to first byte	Time to First Byte (TTFB) refers to the time between the browser requesting a page and when it receives the first byte of information from the server. This time includes DNS lookup and establishing the connection using a TCP handshake and SSL handshake if the request is made over https.
472	Time to interactive	Glossary, Performance, Reference, Web Performance
472	Time to interactive	Time to Interactive (TTI) is a non-standardized web performance 'progress' metric defined as the point in time when the last Long Task finished and was followed by 5 seconds of network and main thread inactivity.
473	TLD	Glossary, Web, WebMechanics
473	TLD	A TLD (top-level domain) is the most generic domain in the Internet's hierarchical DNS (domain name system). A TLD is the final component of a domain name, for example, "org" in `developer.mozilla.org`.
474	Transport Layer Security (TLS)	Cryptography, Glossary, Infrastructure, Security, Web Performance
474	Transport Layer Security (TLS)	Transport Layer Security (TLS), formerly known as SSL, is a protocol used by applications to communicate securely across a network, preventing tampering with and eavesdropping on email, web browsing, messaging, and other protocols. Both SSL and TLS are client / server protocols that ensure communication privacy by using cryptographic protocols to provide security over a network. When a server and client communicate using TLS, it ensures that no third party can eavesdrop or tamper with any message.
475	TOFU	HTTP, SSH, Security
475	TOFU	Trust On First Use (TOFU) is a security model in which a client needs to create a trust relationship with an unknown server. To do that, clients will look for identifiers (for example public keys) stored locally. If an identifier is found, the client can establish the connection. If no identifier is found, the client can prompt the user to determine if the client should trust the identifier.
476	Transmission Control Protocol (TCP)	Glossary, Networking, SSL, Security, TCP, TLS, Web Performance
476	Transmission Control Protocol (TCP)	TCP (Transmission Control Protocol) is a Transport Layer host-to-host protocol for connection-oriented communication between two computers on an IP network. TCP uses virtual ports to create a virtual end-to-end connection that can reuse the physical connections between two computers. TCP encapsulates higher level protocol data such as HTTP and, SMTP (email) .
477	Tree shaking	JavaScript, Modules, Statement, Web Performance, export, import, tree shaking
477	Tree shaking	Tree shaking is a term commonly used within a JavaScript context to describe the removal of dead code.
478	Trident	Browser, Glossary, Infrastructure, Trident
478	Trident	Trident (or MSHTML) was a layout engine that powered Microsoft Internet Explorer. A Trident fork called EdgeHTML replaced Trident in Internet Explorer's successor, Microsoft Edge.
479	Truthy	CodingScripting, Glossary, JavaScript
479	Truthy	In JavaScript, a truthy value is a value that is considered `true` when encountered in a Boolean context. All values are truthy unless they are defined as Falsy (i.e., except for `false`, `0`, `-0`, `0n`, `""`, `null`, `undefined`, and `NaN`).
480	TTL	Caching, Domain Name System, Glossary, Infrastructure, Networking, Performance
480	TTL	TTL can refer either to :
481	TURN	Glossary, Infrastructure, TURN, WebMechanics, WebRTC
481	TURN	TURN (Traversal Using Relays around NAT) is a protocol enabling a computer to receive and send data from behind a NAT (NAT) or firewall. TURN is used by WebRTC to allow any two devices on the Internet to enter a peer-to-peer connection.
482	Type coercion	Coercion, JavaScript, Type coercion
482	Type coercion	Type coercion is the automatic or implicit conversion of values from one data type to another (such as strings to numbers). Type conversion is similar to type coercion because they both convert values from one data type to another with one key difference — type coercion is implicit whereas type conversion can be either implicit or explicit.
483	Type conversion	CodingScripting, Glossary, Type casting, Type conversion
483	Type conversion	Type conversion (or typecasting) means transfer of data from one data type to another. Implicit conversion happens when the compiler automatically assigns data types, but the source code can also explicitly require a conversion to take place. For example, given the instruction `5+2.0`, the floating point `2.0` is implicitly typecasted into an integer, but given the instruction `Number("0x11")`, the string "0x11" is explicitly typecasted as the number 17.
484	Type	CodingScripting, Glossary, JavaScript
484	Type	Type is a characteristic of a value affecting what kind of data it can store, and the structure that the data will adhere to. For example, a `Boolean` Data Type can hold only a `true` or `false` value at any given time, whereas a `String` has the ability to hold a string or a sequence of characters, a `Number` can hold numerical values of any kind, and so on.
485	UDP (User Datagram Protocol)	Glossary, Infrastructure, Networking, Protocols, UDP
485	UDP (User Datagram Protocol)	UDP (User Datagram Protocol) is a long standing protocol used together with IPv6 for sending data when transmission speed and efficiency matter more than security and reliability.
486	UI	Accessibility, Design, Glossary
486	UI	User Interface (UI) is anything that facilitates the interaction between a user and a machine. In the world of computers, it can be anything from a keyboard, a joystick, a screen or a program. In case of computer software, it can be a command-line prompt, a webpage, a user input form, or the front-end of any application.
487	undefined	CodingScripting, Glossary, JavaScript, NeedsContent
487	undefined	`undefined` is a primitive value automatically assigned to variable that have just been declared, or to formal Argument for which there are no actual arguments.
488	Unicode	Infrastructure
488	Unicode	Unicode is a standard Character set that numbers and defines Character from the world's different languages, writing systems, and symbols.
489	URI	Glossary, HTTP, Search, URI, URL
489	URI	A URI (Uniform Resource Identifier) is a string that refers to a resource.
490	URL	Glossary, Infrastructure, l10n:priority
490	URL	Uniform Resource Locator (URL) is a text string that specifies where a resource (such as a web page, image, or video) can be found on the Internet.
491	URN	Glossary, Intro, Navigation, urn
491	URN	URN (Uniform Resource Name) is a URI in a standard format, referring to a resource without specifying its location or whether it exists. This example comes from RFC3986: `urn:oasis:names:specification:docbook:dtd:xml:4.1.2`
492	Usenet	Glossary, WebMechanics
492	Usenet	Usenet is an internet discussion system where each post is duplicated on many servers. The equivalent of Internet forums in its day, Usenet functioned like a bulletin board system.
493	User agent	Browser, Glossary, UA, User-agent, Web Browser, WebMechanics, agent, user agent, userAgent
493	User agent	A user agent is a computer program representing a person, for example, a Browser in a World Wide Web context.
494	UTF-8	CodingScripting, Glossary, HTML, JavaScript, Utf-8
494	UTF-8	UTF-8 (UCS Transformation Format 8) is the World Wide Web's most common Character encoding. Each character is represented by one to four bytes. UTF-8 is backward-compatible with ASCII and can represent any standard Unicode character.
495	UX	Accessibility, Composing, Design, Glossary, Navigation
495	UX	UX is an acronym that stands for User eXperience. It is the study of the interaction between users and a system. Its goal is to make a system easy to interact with from the user's point of view.
496	Validator	Beginner, Glossary, Security
496	Validator	A validator is a program that checks for syntax errors in code. Validators can be created for any format or language, but in our context we speak of tools that check HTML, CSS, and XML.
497	Value	CodingScripting, Glossary, NeedsContent
497	Value	In the context of data or an object Wrapper around that data, the value is the Primitive that the object wrapper contains. In the context of a Variable or Property, the value can be either a primitive or an Object reference.
498	Variable	CodingScripting, Glossary, JavaScript
498	Variable	A variable is a named reference to a Value. That way an unpredictable value can be accessed through a predetermined name.
499	Vendor Prefix	CodingScripting, Glossary
499	Vendor Prefix	Browser vendors sometimes add prefixes to experimental or nonstandard CSS properties and JavaScript APIs, so developers can experiment with new ideas while—in theory—preventing their experiments from being relied upon and then breaking web developers' code during the standardization process. Developers should wait to include the unprefixed property until browser behavior is standardized.
500	Viewport	CodingScripting, Glossary, Layout, viewport
500	Viewport	A viewport represents a polygonal (normally rectangular) area in computer graphics that is currently being viewed. In web browser terms, it refers to the part of the document you're viewing which is currently visible in its window (or the screen, if the document is being viewed in full screen mode). Content outside the viewport is not visible onscreen until scrolled into view.
501	Visual Viewport	Glossary, VisualViewport, viewport, visual viewport
501	Visual Viewport	The portion of the viewport that is currently visible is called the visual viewport. This can be smaller than the layout viewport, such as when the user has pinched-zoomed. The visual viewport is the visual portion of a screen excluding on-screen keyboards, areas outside of a pinch-zoom area, or any other on-screen artifact that doesn't scale with the dimensions of a page.
502	VoIP	Glossary, Infrastructure, VoIP
502	VoIP	VoIP (Voice over Internet Protocol) is a technology used to transmit voice messages over IP (Internet Protocol) networks. Common VoIP packages include Skype, Msn Messenger, Yahoo and many more. Everything transferrred through VoIP is digital. It is also known as IP telephony, or broadband telephony. The main reason for using VoIP technology is because of cost.
503	W3C	Community, Consortium, Glossary, Intro, W3C, Web consortium
503	W3C	The World Wide Web Consortium (W3C) is an international body that maintains World Wide Web rules and frameworks.
504	WAI	Accessibility, Glossary
504	WAI	WAI or Web Accessibility Initiative is an effort by the World Wide Web Consortium (W3C) to improve accessibility for people with various challenges, who may need a nonstandard browser or devices.
505	WCAG	Accessibility, Glossary, WCAG, Web Guidelines
505	WCAG	Web Content Accessibility Guidelines (WCAG) are a recommendation published by the WAI group at the W3C. They outline a set of guidelines for making content accessible primarily for people with disabilities but also for limited-resource devices such as mobile phones.
506	Web performance	Glossary, Perceived Performance, Reference, Web Performance
506	Web performance	Web performance is the objective time from when a request for content is made until the requested content is displayed in the user's browser, objective render times, and the subjective user experience of load time and runtime.
507	Web server	web server, web-server
507	Web server	A web server is a piece of software that often runs on a hardware server offering service to a user, usually referred to as the client. A server, on the other hand, is a piece of hardware that lives in a room full of computers, commonly known as a data center.
508	Web standards	Glossary, Infrastructure, Web Standards, standards, web specifications
508	Web standards	Web standards are rules established by international standards bodies and defining how the World Wide Web works (and sometimes controlling the Internet as well).
509	WebAssembly	Glossary, Infrastructure
509	WebAssembly	WebAssembly (abbr. Wasm) is an open binary programming format that can be run in modern web Browser in order to gain performance and/or provide new features for web pages.
510	WebDAV	Glossary, Infrastructure
510	WebDAV	WebDAV (Web Distributed Authoring and Versioning) is an HTTP Extension that lets web developers update their content remotely from a client.
511	WebExtensions	CodingScripting, Glossary, NeedsContent, WebExtensions
511	WebExtensions	WebExtensions is a cross-browser system for developing browser extensions in Firefox. This system provides APIs, which to a large extent are supported across different browsers like Mozilla Firefox, Google Chrome, Opera Browser, Microsoft Edge, or Apple Safari.
512	WebGL	Advanced, CodingScripting, Glossary, Web Graphics, WebGL
512	WebGL	WebGL (Web Graphics Library) is a JavaScript API that draws interactive 2D and 3D graphics.
513	WebIDL	CodingScripting, Glossary, WebIDL
513	WebIDL	WebIDL is the interface description language used to describe the type, interface, method, property, and other components which make up a Web application programming interface (API). It uses a somewhat stylized syntax which is independent of any specific programming language, so that the underlying code which is used to build each API can be written in whatever language is most appropriate, while still being possible to map the API's components to JavaScript-compatible constructs.
514	WebKit	Browser, Glossary, Intro, Web, WebKit, WebMechanics
514	WebKit	WebKit is a framework that displays properly-formatted webpages based on their markup. Apple Safari depends on WebKit, and so do many mobile browsers (since WebKit is highly portable and customizable).
515	WebM	Composing, Glossary, Infrastructure, WebM
515	WebM	WebM is royalty-free and is an open web video format natively supported in Mozilla Firefox.
516	WebP	Beginner, Composing, Glossary, Infrastructure, WebP
516	WebP	WebP is a lossless and lossy compression image format developed by Google.
517	WebRTC	CodingScripting, Glossary, Infrastructure, JavaScript, P2P, VoIP, Web, WebRTC
517	WebRTC	WebRTC (Web Real-Time Communication) is an API that can be used by video-chat, voice-calling, and P2P-file-sharing Web apps.
518	WebSockets	Connection, Glossary, Infrastructure, Networking, Protocols, Web, WebSocket
518	WebSockets	WebSocket is a protocol that allows for a persistent TCP connection between Server and client so they can exchange data at any time.
519	WebVTT	Audio, CodingScripting, Glossary, Video, Web, WebVTT
519	WebVTT	WebVTT (Web Video Text Tracks) is a W3C specification for a file format marking up text track resources in combination with the HTML `track` element.
520	WHATWG	Community, DOM, Glossary, HTML, HTML5, WHATWG, Web, standards
520	WHATWG	The WHATWG (Web Hypertext Application Technology Working Group) is a community that maintains and develops web standards, including DOM, Fetch, and HTML. Employees of Apple, Mozilla, and Opera established WHATWG in 2004.
521	Whitespace	Glossary, Lexical Grammar, whitespace
521	Whitespace	Whitespace refers to Character which are used to provide horizontal or vertical space between other characters. Whitespace is often used to separate tokens in HTML, CSS, JavaScript, and other computer languages.
522	World Wide Web	Glossary, Infrastructure, WWW, World Wide Web
522	World Wide Web	The World Wide Web—commonly referred to as WWW, W3, or the Web—is an interconnected system of public webpages accessible through the Internet. The Web is not the same as the Internet: the Web is one of many applications built on top of the Internet.
523	Wrapper	CodingScripting, Glossary, Wrapper
523	Wrapper	In programming languages such as JavaScript, a wrapper is a function that is intended to call one or more other functions, sometimes purely for convenience, and sometimes adapting them to do a slightly different task in the process.
524	XForms	CodingScripting, Glossary, Obsolete, XForms
524	XForms	XForms is a convention for building Web forms and processing form data in the XML format.
525	XHR (XMLHttpRequest)	API, Beginner, CodingScripting, Glossary, XMLHttpRequest
525	XHR (XMLHttpRequest)	`XMLHttpRequest` (XHR) is a JavaScript API to create AJAX requests. Its methods provide the ability to send network requests between the browser and a server.
526	XHTML	CodingScripting, Glossary, XHTML
526	XHTML	XHTML is a term that was historically used to describe HTML documents written to conform with XML syntax rules.
527	XInclude	CodingScripting, Glossary
527	XInclude	XML Inclusions (XInclude) is a W3C Recommendation to allow inclusion of XML more different sources in a more convenient fashion than XML external entities. When used in conjunction with XPointer (Firefox supports a subset of it, and is used in the code sample below), XInclude can also target just specific portions of a document for inclusion. Firefox does not support it natively, but the following function aims to allow its use with documents passed into it.
528	XLink	CodingScripting, Glossary
528	XLink	XLink is a W3C standard which is used to describe links between XML and XML or other documents. Some its behaviors are left to the implementation to determine how to handle.
529	XML	CodingScripting, Glossary, XML, l10n:priority
529	XML	eXtensible Markup Language (XML) is a generic markup language specified by the W3C. The information technology (IT) industry uses many languages based on XML as data-description languages.
530	XPath	CodingScripting, Glossary, XML, XPath
530	XPath	XPath is a query language that can access sections and content in an XML document.
531	XQuery	CodingScripting, Glossary, XML, XQuery
531	XQuery	XQuery is a computer language for updating, retrieving, and calculating data in XML databases.
532	XSLT	CodingScripting, Glossary, XML, XSLT
532	XSLT	eXtensible Stylesheet Language Transformations (XSLT) is a declarative language used to convert XML documents into other XML documents, HTML, PDF, plain text, and so on.