Value definition syntax - CSS: Cascading Style Sheets 编辑

CSS value definition syntax, a formal grammar, is used for defining the set of valid values for a CSS property or function. In addition to this syntax, the set of valid values can be further restricted by semantic constraints (for example, for a number to be strictly positive).

The definition syntax describes which values are allowed and the interactions between them. A component can be a keyword, some characters considered as a literal, or a value of a given CSS data type or of another CSS property.

Component value types

Keywords

Generic keywords

A keyword with a predefined meaning appears literally, without quotation marks. For example: auto, smaller or ease-in.

The specific case of inherit, initial and unset

All CSS properties accept the keywords inherit, initial and unset, that are defined throughout CSS. They are not shown in the value definition, and are implicitly defined.

Literals

In CSS, a few characters can appear on their own, like the slash ('/') or the comma (','), and are used in a property definition to separate its parts. The comma is often used to separate values in enumerations, or parameters in mathematical-like functions; the slash often separates parts of the value that are semantically different, but have a common syntax. Typically, the slash is used in shorthand properties; to separate component that are of the same type, but belong to different properties.

Both symbols appear literally in a value definition.

Data types

Basic data types

Some kind of data are used throughout CSS, and are defined once for all values in the specification. Called basic data types, they are represented with their name surrounded by the symbol '<' and '>': <angle>, <string>, …

Non-terminal data types

Less common data types, called non-terminal data types, are also surrounded  by '<' and '>'.

Non-terminal data types are of two kinds:

• data types sharing the same name of a property, put between quotes. In this case, the data type shares the same set of values as the property. They are often used in the definition of shorthand properties.
• data type not sharing the same name of a property. These data types are very close to the basic data types. They only differ from the basic data types in the physical location of their definition. In this case, the definition is usually physically very close to the definition of the property using them.

Component value combinators

Brackets

Brackets enclose several entities, combinators, and multipliers, then transform them as a single component. They are used to group components to bypass the precedence rules.

bold [ thin && <length> ]

This example matches the following values:

• bold thin 2vh
• bold 0 thin
• bold thin 3.5em

But not:

• thin bold 3em, as bold is juxtaposed with the component defined by the brackets, it must appear before it.

Juxtaposition

Placing several keywords, literals or data types, next to one another, only separated by one or several spaces, is called juxtaposition. All juxtaposed components are mandatory and should appear in the exact order.

bold <length> , thin

This example matches the following values:

• bold 1em, thin
• bold 0, thin
• bold 2.5cm, thin
• bold 3vh, thin

But not:

• thin 1em, bold, as the entities must be in the expressed order
• bold 1em thin, as the entities are mandatory; the comma, a literal, must be present
• bold 0.5ms, thin, as the ms values are not <length>

Double ampersand

Separating two or more components, by a double ampersand, &&, means that all these entities are mandatory but may appear in any order.

bold && <length>

This example matches the following values:

• bold 1em
• bold 0
• 2.5cm bold
• 3vh bold

But not:

• bold, as both components must appear in the value.
• bold 1em bold, as both components must appear only one time.

Note: juxtaposition has precedence over the double ampersand, meaning that bold thin && <length> is equivalent to [ bold thin ] && <length>. It describes bold thin <length> or <length> bold thin but not bold <length> thin.

Double bar

Separating two or more components by a double bar, ||, means that all entities are options: at least one of them must be present, and they may appear in any order. Typically this is used to define the different values of a shorthand property.

<'border-width'> || <'border-style'> || <'border-color'>

This example matches the following values:

• 1em solid blue
• blue 1em
• solid 1px yellow

But not:

• blue yellow, as a component must appear at most one single time.
• bold, as it isn't a keyword allowed as value of any of the entity.

Note: the double ampersand has precedence over the double bar, meaning that bold || thin && <length> is equivalent to bold || [ thin && <length> ]. It describes bold, thin <length>, bold thin <length>, or thin <length> bold but not <length> bold thin as bold, if not omitted, must be placed before or after the whole thin && <length> component.

Single bar

Separating two or more entities by a single bar, |, means that all entities are exclusive options: exactly one of these options must be present. This is typically used to separate a list of possible keywords.

<percentage> | <length> | left | center | right | top | bottom

This example matches the following values:

• 3%
• 0
• 3.5em
• left
• center
• right
• top
• bottom

But not:

• center 3%, as only one of the components must be present.
• 3em 4.5em, as a component must be present at most one time.

Note: the double bar has precedence over the single bar, meaning that bold | thin || <length> is equivalent to bold | [ thin || <length> ]. It describes bold, thin, <length>, <length> thin, or thin <length>but not bold <length> as only one entity from each side of the | combinator can be present.

Component value multipliers

A multiplier is a sign that indicate how many times a preceding entity can be repeated. Without a multiplier, an entity must appear exactly one time.

Note that multipliers cannot be added and have all precedence over combinators.

Asterisk (*)

The asterisk multiplier indicates that the entity may appear zero, one or several times.

bold smaller*

This example matches the following values:

• bold
• bold smaller
• bold smaller smaller
• bold smaller smaller smaller, and so on.

But not:

• smaller, as bold is juxtaposed, and must appear before any smaller keyword.

Plus (+)

The plus multiplier indicates that the entity may appear one or several times.

bold smaller+

This example matches the following values:

• bold smaller
• bold smaller smaller
• bold smaller smaller smaller, and so on.

But not:

• bold, as smaller must appear at least one time.
• smaller, as bold is juxtaposed and must appear before any smaller keyword.

Question mark (?)

The question mark multiplier indicates that the entity is optional, and must appear zero or one time.

bold smaller?

This example matches the following values:

• bold
• bold smaller

But not:

• bold smaller smaller, as smaller must appear at most one time.
• smaller, as bold is juxtaposed and must appear before any smaller keyword.

Curly braces ({ })

The curly braces multiplier, enclosing two integers separated by a comma, A and B, indicates that the entity must appear at least A times and at most B times.

bold smaller{1,3}

This example matches the following values:

• bold smaller
• bold smaller smaller
• bold smaller smaller smaller

But not:

• bold, as smaller must appear at least one time.
• bold smaller smaller smaller smaller, as smaller must appear at most three times.
• smaller, as bold is juxtaposed and must appear before any smaller keyword

Hash mark (#)

The hash mark multiplier indicates that the entity may be repeated one or more times (for example, the plus multiplier), but each occurrence is separated by a comma (',').

bold smaller#

This example matches the following values:

• bold smaller
• bold smaller, smaller
• bold smaller, smaller, smaller, and so on.

But not:

• bold, as smaller must appear at least one time.
• bold smaller smaller smaller, as the different occurrence of smaller must be separated by commas.
• smaller, as bold is juxtaposed and must appear before any smaller keyword.

Exclamation point (!)

The exclamation point multiplier after a group indicates that the group is required, and must produce at least one value; even if the grammar of the items within the group would otherwise allow the entire contents to be omitted, at least one component value must not be omitted.

[ bold? smaller? ]!

This example matches the following values:

• bold
• smaller
• bold smaller

But not:

• neither bold nor smaller, as one of them must appear.
• smaller bold, as bold is juxtaposed and must appear before the smaller keyword.
• bold smaller bold, as bold and smaller may only appear once.

Summary

Specifications

See also

• CSS Key Concepts: CSS syntax, at-rule, comments, specificity and inheritance, the box, layout modes and visual formatting models, and margin collapsing, or the initial, computed, resolved, specified, used, and actual values. Definitions of value syntax, shorthand properties and replaced elements.