CSS Custom Properties for Cascading Variables Module Level 1

Status: ED
Shortname: css-variables
TR: http://www.w3.org/TR/css-variables/
ED: http://dev.w3.org/csswg/css-variables/
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.

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 variables, 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.

Module Interactions

This module defines a new type of primitive value, the variable, which is accepted by all properties.

Values

This specification follows the CSS property definition conventions from [[!CSS21]]. Value types not defined in this specification are defined in CSS Level 2 Revision 1 [[!CSS21]]. Other CSS modules may expand the definitions of these value types: for example if the <> type was used in this specification, then [[CSS3COLOR]] would expand its definition.

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

This specification defines an open-ended set of properties called custom properties, which are used to define variables. Name: var-* Values: [ <<value>> | <<CDO>> | <<CDC>> ]+ 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 A custom property is any property whose name starts with "var-", and contains at least one additional character. Custom properties are solely for use by authors and users; CSS will never give them a meaning beyond what is presented here. Note: Authors are recommended to choose custom property names so that the part after the "var-" is an identifier by itself, so that it can be referenced without character escaping. For example, 'var-0' needs to be referenced as ''var(\30)'', because ''0'' isn't a valid identifier. (U+0030 is the hexadecimal code for the ''0'' character.)
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 {
			var-main-color: #06c;
			var-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. The "var-" prefix must be written in lower-case.
For example, ''VAR-FOO'' is invalid, because the prefix isn't "var-". While both ''var-foo'' and ''var-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 extremely permissive in their value, accepting any value that the core syntax of CSS allows, plus the CDO and CDC tokens (written "<!--" and "-->"). The <value> in its grammar corresponds to the "value" production in CSS 2.1 Chapter 4.1 [[!CSS21]], while <CDO> and <CDC> correspond to the tokens of the same name from the same chapter.

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.

For example, the following is a valid custom property: 
		var-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 variables 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()''. The primary purpose of custom properties is to define cascading variables. In CSS, a cascading variable is a value that can be substituted into other properties, allowing authors to "abstract" parts of their page's CSS out and reuse it in several places. Every custom property defines a corresponding variable with the same name, minus the "var-" prefix. For example, the custom property 'var-foo' defines a variable named ''foo''. See the next chapter for details on how to use variables. Note: Custom properties can be put to several other uses, of course. For example, they can be used to conveniently attach values to elements so that JavaScript can later use those values. Another example is providing "custom CSS" by treating "var-" as a kind of "author prefix" (similar to a vendor prefix) that allows an author to write custom CSS properties without having them thrown away as invalid by the CSS parser, and then having JavaScript come along afterward to actually implement the functionality.
This style rule: 
		:root {
			var-header-color: #06c;
		}
declares a custom property named "var-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 via the "header-color" variable: 
		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 'var-header-color' property on the root element.
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..
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 { var-color: blue; }
		div { var-color: green; }
		#alert { var-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>
Custom properties may use variables in their own values to build up composite variables. This can create cyclic dependencies where two or more custom properties each attempt to use the variable that the other defines; doing so makes all the custom properties involved in the cycle compute to their initial value (which is a guaranteed-invalid value).
This example shows a custom property safely using a variable: 
		:root {
			var-main-color: #c06;
			var-accent-background: linear-gradient(to top, var(main-color), white);
		}
The 'var-accent-background' property (along with any other properties that use ''var(main-color)'') will automatically update when the 'var-main-color' property is changed.
On the other hand, this example shows an invalid instance of variables depending on each other: 
		:root {
			var-one: calc(var(two) + 20px);
			var-two: calc(var(one) - 20px);
		}
Both 'var-one' and 'var-two' now define invalid variables rather than lengths.
It is important to note that custom properties resolve any variables 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   { var-foo: 10px; }
		two   { var-bar: calc(var(foo) + 10px); }
		three { var-foo: calc(var(bar) + 10px); }
The <one> element defines a value for 'var-foo'. The <two> element inherits this value, and additionally assigns a value to 'var-bar' using the ''foo'' variable. Finally, the <three> element inherits the 'var-bar' value after variable substitution (in other words, it sees the value ''calc(10px + 10px)''), and then redefines 'var-foo' in terms of that value. Since the value it inherited for 'var-bar' no longer contains a reference to the 'var-foo' property defined on <one>, defining 'var-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

Every custom property automatically defines a corresponding cascading variable, which can then be substituted into another property with the ''var()'' function. The syntax of ''var()'' is: 
	<variable> = var( variable-name [, <> ]? )
	<fallback> = [ <> | <> | <> ]+
A variable can be used in place of any part of a value in any property on an element. Variables 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 <> value is identical to the syntax of a custom property value: it accepts anything allowed by the core syntax of CSS (except that, since it's a value in a function rather than a value in a declaration, some tokens that were allowed in a custom property don't work in a <> and vice versa, because they'd close the function early). Note: The syntax of <>, like that of custom properties, allows commas. For example, a variable use like ''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 <> value. A property value containing a variable must be assumed to be valid at parse time. It is only syntax-checked at computed-value time, after variable references have been resolved. To resolve a variable in a property's value:
  1. If the variable named by the first argument to the ''var()'' function is a valid variable, replace the ''var()'' function by the value of the corresponding custom property.
  2. Otherwise, if the ''var()'' function has a <> value as its second argument, replace the ''var()'' function by the <> value. If there are any ''var()'' references in the fallback, resolve them as well.
  3. 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 {
			var-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 {
			var-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 {
			var-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 {
		  var-gap: 20;
		  margin-top: calc(var(gap) * 1px);
		}
A variable is substituted for its value in the property value at computed-value time. If a declaration, once all variables 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 { var-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.
In some cases, it can be useful to provide a "default" value for a variable in case the variable isn't defined or is invalid. For example, if a site uses variables to provide "hooks" for customization, expecting the variables to be defined in a separate custom stylesheet, the main stylesheet can use default values for its variable so that the theming stylesheet can just override the variables it cares about, rather than being forced to provide values for all of them.

Invalid Variables

When a custom property has its initial value, the variable it defines represents an invalid variable. Using an invalid variable in a property value (including other custom properties) makes the declaration invalid at computed-value time. A declaration can be invalid at computed-value time if it uses an invalid variable, as explained above, or if it uses a valid variable, but the property value, after substituting its variables, 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.
For example, in the following code: 
		:root { var-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 variable itself was invalid. 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.

APIs

Extensions to the CSSStyleDeclaration Interface

The CSSStyleDeclaration interface is amended as follows: 
	partial interface CSSStyleDeclaration {
		readonly attribute CSSVariablesDeclaration var;
	}
A CSSStyleDeclaration is the associated style declaration for the CSSVariablesDeclaration assigned to its var attribute. While the CSSStyleDeclaration interface normally contains attributes that are camel-cased name variants of all CSS properties (and sometimes also attributes for their canonical names), it must not contain any such attributes for custom properties. The camel-case trick does not work, as custom property names are case-sensitive, and there are potentially an infinity of custom properties, which is incompatible with the normal behavior of exposing every property whether it was set in the corresponding declaration block or not.

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.

The CSSVariablesDeclaration Interface

The CSSVariablesDeclaration interface exposes the custom properties declared in the parent declaration block that have a non-initial value. 
	interface CSSVariablesDeclaration {
		getter DOMString (DOMString varName);
		setter creator void (DOMString varName, DOMString varValue);
		deleter void (DOMString varName);
	}
The supported property names on a CSSStyleDeclaration object are the property names of all the custom properties in the CSS declaration block declarations with a non-initial value, with the "var-" prefix removed. Before running any of the algorithms in this section, prepend "var-" to varName's value. When asked to get the value of a variable, if varName is in the CSS declaration block declarations, invoke getPropertyValue() on the associated style declaration by passing varName as its argument, and return the returned value. Otherwise, return the empty string. When asked to set or create the value of a variable, invoke setProperty() on the associated style declaration by passing varName as the first argument and varValue as the second argument.

Note that using setProperty() to set a property to the empty string instead deletes the property. When asked to delete the value of a variable, if varName matches the grammar of a custom property name, invoke removeProperty() on the associated style declaration by passing varName as its argument, and return the returned value. Otherwise, do nothing and return the empty string.

For example, given the following style sheet: 
		div {
			var-foo: 16px;
			var-Bar: red;
			var-foo-bar: 50%;
		}
Here are the results of several JavaScript expressions, assuming that el is a JavaScript variable holding an element that the above style rule applies to:
Code Value Notes
el.style.var.foo "16px" The value of 'var-foo'.
el.style.var.Bar "red" The value of 'var-Bar'.
el.style.var["foo-bar"] "50%" The value of 'var-foo-bar'.
el.style.varFoo n/a Custom properties don't exist directly on "style"
el.style.varBar n/a Not even if the casing matches.
el.style.var.foo-bar NaN Retrieves the value of 'var-foo' (the string "16px") and subtracts some unrelated JavaScript variable named "bar" from it, rather than just retrieving the value of 'var-foo-bar'.
Iterating over all of the custom properties (for example, for a JS library to find the ones it knows about and wants to respond to) is also simple with the var property: 
		var customProps = el.style.var;
		for(customPropName in customProps) {
			if( knownCustomPropName(customPropName) ) {
				var customPropValue = customProps[customPropName];
				/* Processing code here. */
			}
		}

Changes from 10 April 2012 Working Draft

  • The value syntax for custom properties has been nailed down more precisely.
  • Case-sensitivity of custom property names has been defined.
  • The fallback argument was added to the var() function.
  • A property that is invalid at computed-value time now either goes inherit or initial, rather than always initial.
  • CSSVariableComponentValue interface has been dropped, pending the *ComponentValue interfaces being created at all.
  • The CSSVariablesDeclaration interface has been added, which stores all the variables defined by a style rule.

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.