Value definition syntax

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.

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 components that are of the same type, but belong to different properties.

Both symbols appear literally in a value definition.

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.

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.

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>

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.

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.

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.

A multiplier is a sign that indicates 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 precedence over all combinators.

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.

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.

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.

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

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 occurrences of smaller must be separated by commas.
• smaller, as bold is juxtaposed and must appear before any smaller keyword.

The hash mark may optionally be followed by curly braces to indicate precisely how many times the repetition occurs.

bold smaller#{1,3}

This example matches the following values:

• bold smaller
• bold smaller, smaller
• bold smaller, smaller, smaller

But not:

• bold smaller, smaller, smaller, smaller, as smaller must appear at most three times.

bold smaller#{2}

This example matches the following value:

• bold smaller, smaller

But not:

• bold smaller, as smaller must appear exactly two times.

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.

• CSS key concepts:
  • CSS syntax
  • Comments
  • Specificity
  • Inheritance
  • Box model
  • Layout modes
  • Visual formatting models
  • Margin collapsing
  • Values
    • Initial values
    • Computed values
    • Used values
    • Actual values
  • Value definition syntax
  • Shorthand properties
  • Replaced elements