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 ' ':, Non-terminal data types Less common data types, called non-terminal data types, are also surrounded by ' '. 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 && 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.

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All juxtaposed components are mandatory and should appear in the exact order. Bold, 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 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 && 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 && is equivalent to bold thin &&. It describes bold thin or bold thin but not bold 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. 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 && is equivalent to bold thin &&.

It describes bold, thin, bold thin, or thin bold but not bold thin as bold, if not omitted, must be placed before or after the whole thin && 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. 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.

This feature replaces a sequence of glyphs with a single glyph, called a ligature, which is preferred for typographic purposes. When enabled, this feature inserts the ligatures which the designer/manufacturer judges should be used in normal conditions.

The ligatures feature is enabled by default, which means it should work without you having to write any CSS — but it’s not enabled by default in Chrome. Furthermore, it cannot be disabled in Safari (Mac or iOS). And some versions of Chrome and Firefox have trouble with ligatures that contain spaces.

This feature, in specified situations, replaces default glyphs with alternate forms which provide better joining behavior. Like ligatures (though not strictly a ligature feature), contextual alternates are commonly used to harmonize the shapes of glyphs with the surrounding context. The contextual alternates feature is enabled by default, which means it should work without you having to write any CSS — but it’s not enabled by default in Chrome. Furthermore, it cannot be disabled in Safari (Mac or iOS). The stylistic sets feature is disabled by default. To enable it with font-feature-settings, use the “ss##” feature tag, where “##” is any two-digit combination from 01–20. Note that fonts employ stylistic sets in completely arbitrary and custom ways.

For example, two different fonts may employ the same feature (like ss01) to replace totally different sets of glyphs; or, a particular font may use ss01 and ss03 to replace sets of glyphs, but ignore ss02. Some type foundries offer documentation about how features are organized. For now, we’ll skip the font-variant property because its “named” values (like styleset(sharp-serifs)) are mapped to numeric stylistic set indices by font-feature-value CSS definitions, which are not yet supported in any major browser.

The ruby notation forms feature is disabled by default. To enable it with the font-variant property or font-variant-east-asian subproperty, use the value ruby. To achieve the same effect with font-feature-settings, use the “ruby” feature tag. Note the rt element that we’re styling, in the code sample above. Is explained in the W3C’s HTML5 specification, in the section on text-level semantics.

Also note that the position and size of ruby text will vary across browsers/versions; consider disabling browser defaults with a and sizing/positioning ruby text manually. This feature adjusts the amount of space between glyphs, generally to provide optically consistent spacing between glyphs. Although a well-designed typeface has consistent inter-glyph spacing overall, some glyph combinations require adjustment for improved legibility.

Besides standard adjustment in the vertical direction, this feature can supply size-dependent kerning data via device tables, “cross-stream” kerning in the X text direction, and adjustment of glyph placement independent of the advance adjustment. Note that this feature may apply to runs of more than two glyphs, and would not be used in monospaced fonts. Also note that this feature applies only to text set vertically. This feature replaces default forms with variants adjusted for vertical writing when in vertical writing mode. While most CJKV glyphs remain vertical when set in vertical writing mode, some take a different form (usually rotated and repositioned) for this purpose. Glyphs covered by this feature correspond to the set normally rotated in low-end DTP applications. (.) The vertical alternates feature is enabled by default, which means it should work without you having to write any CSS — but it’s not enabled by default in any version of IE or Firefox.

Furthermore, it cannot be enabled at all in Safari (Mac or iOS). To minimize the number of glyph alternates, it is sometimes desired to decompose a character into two glyphs.

Additionally, it may be preferable to compose two characters into a single glyph for better glyph processing. This feature permits such composition/decompostion.

The feature should be processed as the first feature processed, and should be processed only when it is called. (.) The glyph composition/decomposition feature is enabled by default, which means it should work without you having to write any CSS — but there are a few tricky conditions around making sure it is enabled. This feature is not enabled by default in Chrome or Safari, but using any font-feature-settings value will enable it. Many scripts used to write multiple languages over wide geographical areas have developed localized variant forms of specific letters, which are used by individual literary communities. For example, a number of letters in the Bulgarian and Serbian alphabets have forms distinct from their Russian counterparts and from each other.

In some cases the localized form differs only subtly from the script 'norm', in others the forms are radically distinct. This feature enables localized forms of glyphs to be substituted for default forms. (.) The localized forms feature is enabled by default, which means it should work without you having to write any CSS — but it’s only actually enabled by default in Firefox, and not supported at all in Safari (Mac or iOS) or on Android.