CSS Custom Properties for Cascading Variables Module Level 1

!Date: 2014-05-06
Status: ED
Shortname: css-variables
Level: 1
Group: csswg
TR: http://www.w3.org/TR/css-variables-1/
ED: http://dev.w3.org/csswg/css-variables/
Previous Version: http://www.w3.org/TR/2013/WD-css-variables-1-20130620/
Previous Version: http://www.w3.org/TR/2013/WD-css-variables-20130312/
Previous Version: http://www.w3.org/TR/2012/WD-css-variables-20120410/
Editor: Tab Atkins Jr., Google, http://xanthir.com/contact
Abstract: This module introduces cascading variables as a new primitive value type that is accepted by all CSS properties, and custom properties for defining them.
Ignored Terms: case-insensitive, getpropertyvalue()
Link Defaults: css-color-3 (property) color, cssom-1 (interface) cssstyledeclaration

Introduction

This section is not normative. Large documents or applications (and even small ones) can contain quite a bit of CSS. Many of the values in the CSS file will be duplicate data; for example, a site may establish a color scheme and reuse three or four colors throughout the site. Altering this data can be difficult and error-prone, since it's scattered throughout the CSS file (and possibly across multiple files), and may not be amenable to Find-and-Replace. This module introduces a family of custom author-defined properties known collectively as custom properties, which allow an author to assign arbitrary values to a property with an author-chosen name, and the ''var()'' function, which allow an author to then use those values in other properties elsewhere in the document. This makes it easier to read large files, as seemingly-arbitrary values now have informative names, and makes editing such files much easier and less error-prone, as one only has to change the value once, in the custom property, and the change will propagate to all uses of that variable automatically.

Defining Custom Properties: the '--*' family of properties

This specification defines an open-ended set of properties called custom properties, which, among other things, are used to define the substitution value of ''var()'' functions.

	Name: --*
	Value: <>
	Initial: (nothing, see prose)
	Applies to: all elements
	Inherited: yes
	Computed value: specified value with variables substituted (but see prose for "invalid variables")
	Media: all
	Animatable: no

A custom property is any property whose name starts with two dashes (U+002D HYPHEN-MINUS), like '--foo'. The <custom-property-name> production corresponds to this: it's defined as any valid identifier that starts with two dashes. Custom properties are solely for use by authors and users; CSS will never give them a meaning beyond what is presented here.

Custom properties define variables, referenced with the ''var()'' notation, which can be used for many purposes. For example, a page that consistently uses a small set of colors in its design can store the colors in custom properties and use them with variables:

		:root {
			--main-color: #06c;
			--accent-color: #006;
		}
		/* The rest of the CSS file */
		#foo h1 {
			color: var(--main-color);
		}

The naming provides a mnemonic for the colors, prevents difficult-to-spot typos in the color codes, and if the theme colors are ever changed, focuses the change on one simple spot (the custom property value) rather than requiring many edits across all stylesheets in the webpage.

Unlike other CSS properties, custom property names are case-sensitive.

While both '--foo' and --FOO are valid, they are distinct properties - using ''var(--foo)'' will refer to the first one, while using ''var(--FOO)'' will refer to the second.

Custom properties are not reset by the 'all' property. We may define a property in the future that resets all variables. The CSS-wide keywords can be used in custom properties, with the same meaning as in any another property. Note: That is, they're interpreted at cascaded-value time as normal, and are not preserved as the custom property's value, and thus are not substituted in by the corresponding variable. Note: While this module focuses on the use of custom properties with the ''var()'' function to create “variables”, they can also be used as actual custom properties, parsed by and acted on by script. It's expected that the CSS Extensions spec [[CSS-EXTENSIONS]] will expand on these use-cases and make them easier to do.

Custom Property Value Syntax

The allowed syntax for custom properties is extremely permissive. The <any-value> production matches any sequence of one or more tokens, so long as the sequence does not contain <>, <>, unmatched <<)-token>>, <<]-token>>, or <<}-token>>, or top-level <> tokens or <> tokens with a value of "!". Note: This definition, along with the general CSS syntax rules, implies that a custom property value never includes an unmatched quote or bracket, and so cannot have any effect on larger syntax constructs, like the enclosing style rule, when reserialized. Note: Custom properties can contain a trailing ''!important'', but this is automatically removed from the property's value by the CSS parser, and makes the custom property "important" in the CSS cascade. In other words, the prohibition on top-level "!" characters does not prevent ''!important'' from being used, as the ''!important'' is removed before syntax checking happens. Note: While <> must represent at least one token, that one token may be whitespace. This implies that ''--foo: ;'' is valid, and the corresponding ''var(--foo)'' call would have a single space as its substitution value, but ''--foo:;'' is invalid.

For example, the following is a valid custom property:

		--foo: if(x > 5) this.width = 10;

While this value is obviously useless as a variable, as it would be invalid in any normal property, it might be read and acted on by JavaScript.

The values of custom properties, and the values of ''var()'' functions substituted into custom properties, are case-sensitive, and must be preserved in their original author-given casing. (Many CSS values are case-insensitive, which user agents can take advantage of by "canonicalizing" them into a single casing, but that isn't allowed for custom properties.) The initial value of a custom property is an empty value; that is, nothing at all. This initial value has a special interaction with the ''var()'' notation, which is explained in the section defining ''var()''. Custom properties are ordinary properties, so they can be declared on any element, are resolved with the normal inheritance and cascade rules, can be made conditional with ''@media'' and other conditional rules, can be used in HTML's style attribute, can be read or set using the CSSOM, etc. Notably, they can even be transitioned or animated, but since the UA has no way to interpret their contents, they always use the "flips at 50%" behavior that is used for any other pair of values that can't be intelligently interpolated. However, any custom property used in a ''@keyframes'' rule becomes animation-tainted, which affects how it is treated when referred to via the ''var()'' function in an animation property.

This style rule:

		:root {
			--header-color: #06c;
		}

declares a custom property named '--header-color' on the root element, and assigns to it the value "#06c". This property is then inherited to the elements in the rest of the document. Its value can be referenced with the ''var()'' function:

		h1 { background-color: var(--header-color); }

The preceding rule is equivalent to writing ''background-color: #06c;'', except that the variable name makes the origin of the color clearer, and if ''var(--header-color)'' is used on other elements in the document, all of the uses can be updated at once by changing the '--header-color' property on the root element.

If a custom property is declared multiple times, the standard cascade rules help resolve it. Variables always draw from the computed value of the associated custom property on the same element:

		:root { --color: blue; }
		div { --color: green; }
		#alert { --color: red; }
		* { color: var(--color); }

		<p>I inherited blue from the root element!</p>
		<div>I got green set directly on me!</div>
		<div id='alert'>
			While I got red set directly on me!
			<p>I'm red too, because of inheritance!</p>
		</div>

Resolving Dependency Cycles

Custom properties are left almost entirely unevaluated, except that they allow and evaluate the ''var()'' function in their value. This can create cyclic dependencies where a custom property uses a ''var()'' referring to itself, or two or more custom properties each attempt to refer to each other. For each element, create an undirected dependency graph, containing nodes for each custom property. If the value of a custom property prop contains a ''var()'' function referring to the property var (including in the fallback argument of ''var()''), add an edge between prop and the var. Edges are possible from a custom property to itself. If there is a cycle in the dependency graph, all the custom properties in the cycle must compute to their initial value (which is a guaranteed-invalid value).

This example shows a custom property safely using a variable:

		:root {
			--main-color: #c06;
			--accent-background: linear-gradient(to top, var(--main-color), white);
		}

The '--accent-background' property (along with any other properties that use ''var(--main-color)'') will automatically update when the '--main-color' property is changed.

On the other hand, this example shows an invalid instance of variables depending on each other:

		:root {
			--one: calc(var(--two) + 20px);
			--two: calc(var(--one) - 20px);
		}

Both '--one' and '--two' now compute to their initial value, rather than lengths.

It is important to note that custom properties resolve any ''var()'' functions in their values at computed-value time, which occurs before the value is inherited. In general, cyclic dependencies occur only when multiple custom properties on the same element refer to each other; custom properties defined on elements higher in the element tree can never cause a cyclic reference with properties defined on elements lower in the element tree.

For example, given the following structure, these custom properties are not cyclic, and all define valid variables:

		<one><two><three /></two></one>
		one   { --foo: 10px; }
		two   { --bar: calc(var(--foo) + 10px); }
		three { --foo: calc(var(--bar) + 10px); }

The <one> element defines a value for '--foo'. The <two> element inherits this value, and additionally assigns a value to '--bar' using the ''foo'' variable. Finally, the <three> element inherits the '--bar' value after variable substitution (in other words, it sees the value ''calc(10px + 10px)''), and then redefines '--foo' in terms of that value. Since the value it inherited for '--bar' no longer contains a reference to the '--foo' property defined on <one>, defining '--foo' using the ''var(--bar)'' variable is not cyclic, and actually defines a value that will eventually (when referenced as a variable in a normal property) resolve to ''30px''.

Using Cascading Variables: the ''var()'' notation

The value of a custom property can be substituted into the value of another property with the ''var()'' function. The syntax of ''var()'' is:

	var() = var( <> [, <> ]? )

The ''var()'' function can be used in place of any part of a value in any property on an element. The ''var()'' function can not be used as property names, selectors, or anything else besides property values. (Doing so usually produces invalid syntax, or else a value whose meaning has no connection to the variable.) The first argument to the function is the name of the custom property to be substituted. The second argument to the function, if provided, is a fallback value, which is used as the substitution value when the referenced custom property is invalid. Note: The syntax of the fallback, like that of custom properties, allows commas. For example, ''var(--foo, red, blue)'' defines a fallback of ''red, blue''; that is, anything between the first comma and the end of the function is considered a fallback value. If a property contains one or more ''var()'' functions, and those functions are syntactically valid, the entire property's grammar must be assumed to be valid at parse time. It is only syntax-checked at computed-value time, after ''var()'' functions have been substituted. To substitute a var() in a property's value:

If the custom property named by the first argument to the ''var()'' function is animation-tainted, and the ''var()'' function is being used in the 'animation' property or one of its longhands, treat the custom property as having its initial value for the rest of this algorithm.
If the value of the custom property named by the first argument to the ''var()'' function is anything but the initial value, replace the ''var()'' function by the value of the corresponding custom property.
Otherwise, if the ''var()'' function has a fallback value as its second argument, replace the ''var()'' function by the fallback value. If there are any ''var()'' references in the fallback, substitute them as well.
Otherwise, the property containing the ''var()'' function is invalid at computed-value time. Note: Other things can also make a property invalid at computed-value time.

The fallback value allows for some types of defensive coding. For example, an author may create a component intended to be included in a larger application, and use variables to style it so that it's easy for the author of the larger application to theme the component to match the rest of the app. Without fallback, the app author must supply a value for every variable that your component uses. With fallback, the component author can supply defaults, so the app author only needs to supply values for the variables they wish to override.

		/* In the component's style: */
		.component .header {
			color: var(--header-color, blue);
		}
		.component .text {
			color: var(--text-color, black);
		}

		/* In the larger application's style: */
		.component {
			--text-color: #080;
			/* header-color isn't set,
			   and so remains blue,
			   the fallback value */
		}

For example, the following code incorrectly attempts to use a variable as a property name:

		.foo {
			--side: margin-top;
			var(--side): 20px;
		}

This is not equivalent to setting ''margin-top: 20px;''. Instead, the second declaration is simply thrown away as a syntax error for having an invalid property name. Similarly, you can't build up a single token where part of it is provided by a variable:

		.foo {
			--gap: 20;
			margin-top: var(--gap)px;
		}

Again, this is not equivalent to setting ''margin-top: 20px;'' (a length). Instead, it's equivalent to ''margin-top: 20 px;'' (a number followed by an ident), which is simply an invalid value for the 'margin-top' property. Note, though, that ''calc()'' can be used to validly achieve the same thing, like so:

		.foo {
		  --gap: 20;
		  margin-top: calc(var(--gap) * 1px);
		}

''var()'' functions are substituted at computed-value time. If a declaration, once all ''var()'' functions are substituted in, is invalid, the declaration is invalid at computed-value time.

For example, the following usage is fine from a syntax standpoint, but results in nonsense when the variable is substituted in:

		:root { --looks-valid: 20px; }
		p { background-color: var(--looks-valid); }

Since ''20px'' is an invalid value for 'background-color', this instance of the property computes to ''transparent'' (the initial value for 'background-color') instead. If the property was one that's inherited by default, such as 'color', it would compute to the inherited value rather than the initial value.

Invalid Variables

When a custom property has its initial value, ''var()'' functions cannot use it for substitution. Attempting to do so makes the declaration invalid at computed-value time, unless a valid fallback is specified. A declaration can be invalid at computed-value time if it contains a ''var()'' that references a custom property with its initial value, as explained above, or if it uses a valid custom property, but the property value, after substituting its ''var()'' functions, is invalid. When this happens, the computed value of the property is either the property's inherited value or its initial value depending on whether the property is inherited or not, respectively, as if the property's value had been specified as the ''unset'' keyword.

For example, in the following code:

		:root { --not-a-color: 20px; }
		p { background-color: red; }
		p { background-color: var(--not-a-color); }

the <p> elements will have transparent backgrounds (the initial value for 'background-color'), rather than red backgrounds. The same would happen if the custom property itself was unset, or contained an invalid ''var()'' function. Note the difference between this and what happens if the author had just written ''background-color: 20px'' directly in their stylesheet - that would be a normal syntax error, which would cause the rule to be discarded, so the ''background-color: red'' rule would be used instead.

Note: The invalid at computed-value time concept exists because variables can't "fail early" like other syntax errors can, so by the time the user agent realizes a property value is invalid, it's already thrown away the other cascaded values.

Variables in Shorthand Properties

The use of ''var()'' functions in shorthand properties presents some unique difficulties. Ordinarily, the value of a shorthand property is separated out into its component longhand properties at parse time, and then the longhands themselves participate in the cascade, with the shorthand more-or-less discarded. If a ''var()'' functions is used in a shorthand, however, one can't tell what values are meant to go where; it may in fact be impossible to separate it out at parse time, as a single ''var()'' function may substitute in the value of several longhands at once. To get around this, implementations must fill in longhands with a special, unobservable-to-authors pending-substitution value that indicates the shorthand contains a variable, and thus the longhand's value is pending variable substitution. This value must then be cascaded as normal, and at computed-value time, after ''var()'' functions are finally substituted in, the shorthand must be parsed and the longhands must be given their appropriate values at that point. Pending-substitution values must be serialized as the empty string, if an API allows them to be observed. Similarly, while [[CSSOM]] defines that shorthand properties are serialized by appropriately concatenating the values of their corresponding longhands, shorthands that are specified with explicit ''var()'' functions must serialize to the original, ''var()''-containing value. For other shorthands, if any of the longhand subproperties for that shorthand have pending-substitution values then the serialized value of the shorthand must be the empty string.

APIs

All custom property declarations have the case-insensitive flag set. Note: Custom properties do not appear on a CSSStyleDeclaration object in camel-cased form, because their names may have both upper and lower case letters which indicate distinct custom properties. The sort of text transformation that automatic camel-casing performs is incompatible with this. They can still be accessed by their proper name via getPropertyValue()/etc.

Serializing Custom Properties

Custom property names must be serialized with the casing as provided by the author.

Ordinarily, property names are restricted to the ASCII range and are case-insensitive, so implementations typically serialize the name lowercased.

Changes since the June 20 2013 Last Call Working Draft

The syntax of custom properties was changed from ''var-foo'' to ''--foo''.
The ''var()'' function takes the full custom property name as its first argument, rather than the name minus a prefix.
The CSSVariablesMap interface was removed, in favor of a better custom-property API to be defined in the future.

Acknowledgments

Many thanks to several people in the CSS Working Group for keeping the dream of variables alive over the years, particularly Daniel Glazman and David Hyatt. Thanks to multiple people on the mailing list for helping contribute to the development of this incarnation of variables, particularly Brian Kardell, David Baron, François Remy, Roland Steiner, and Shane Stephens.