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

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?".)

Semantics in JavaScript

In JavaScript, consider a function that takes a string parameter, and returns an <li> element with that string as its textContent. Would you need to look at the code to understand what the function did if it was called build('Peach'), or createLiWithContent('Peach')?

Semantics in CSS

In CSS, consider styling a list with li elements representing different types of fruits. Would you know what part of the DOM is being selected with div > ul > li, or .fruits__item?

Semantics in HTML

In HTML, for example, the <h1> element is a semantic element, which gives the text it wraps around the role (or meaning) of "a top level heading on your page."

<h1>This is a top level heading</h1>

By default, most browser's user agent stylesheet will style an <h1> with a large font size to make it look like a heading (although you could style it to look like anything you wanted).

On the other hand, you could make any element look like a top level heading. Consider the following:

<span style="font-size: 32px; margin: 21px 0;">Is this a top level heading?</span>

This will render it to look like a top level heading, but it has no semantic value, so it will not get any extra benefits as described above. It is therefore a good idea to use the right HTML element for the right job.

HTML should be coded to represent the data that will be populated and not based on its default presentation styling. Presentation (how it should look), is the sole responsibility of CSS.

Some of the benefits from writing semantic markup are as follows:

Search engines will consider its contents as important keywords to influence the page's search rankings (see SEO)
Screen readers can use it as a signpost to help visually impaired users navigate a page
Finding blocks of meaningful code is significantly easier than searching through endless divs with or without semantic or namespaced classes
Suggests to the developer the type of data that will be populated
Semantic naming mirrors proper custom element/component naming

When approaching which markup to use, ask yourself, "What element(s) best describe/represent the data that I'm going to populate?" For example, is it a list of data?; ordered, unordered?; is it an article with sections and an aside of related information?; does it list out definitions?; is it a figure or image that needs a caption?; should it have a header and a footer in addition to the global site-wide header and footer?; etc.