Overview.src.html

<h1>CSS Custom Properties for Cascading Variables Module Level 1</h1>

<pre class='metadata'>
Status: LCWD
Shortname: css-variables
Level: 1
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-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 Properties: var-0, var-foo, var-main-color, color, var-Bar, var-two, margin-top, var-header-color, var-accent-background, var-foo-bar, var-bar, background-color, var-one, all
Ignored Terms: <color>, <bad-url>, <bad-string>, <semicolon>
Date: 2013-05-16
Deadline: 2013-06-13
</pre>

<h2>
Introduction</h2>

	<em>This section is not normative.</em>

	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 <a>custom properties</a>,
	which allow an author to assign arbitrary values to a property with an author-chosen name,
	and <a>variables</a>,
	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 <a>custom property</a>,
	and the change will propagate to all uses of that variable automatically.


<h3>
Module Interactions</h3>

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

<h3>
Values</h3>

	This specification follows the <a href="http://www.w3.org/TR/CSS21/about.html#property-defs">CSS property definition conventions</a> 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 <<color>> type was used in this specification,
	then [[CSS3COLOR]] would expand its definition.


<h2 id='defining-variables'>
Defining Custom Properties: the 'var-*' family of properties</h2>

	This specification defines an open-ended set of properties called <a>custom properties</a>,
	which are used to define <a>variables</a>.

	<pre class='propdef'>
	Name: var-*
	Value: <<any-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
	</pre>

	<p class='issue'>
		Should variables have special animation behavior, 
		such that they just never animate,
		not even with the "just flip at 50% progress" like everything else?

	<p class='issue'>
		Should the global values initial/inherit/default be interpreted?
		I'm guessing no,
		because we're not meant to interpret *anything*.
		You can get switch it back to inheriting by setting it to a guaranteed-invalid value,
		like ''var-foo: var(foo);'',
		but you can't get ''default'' behavior.

	<p class='issue'>
		What's the relationship of custom properties and the 'all' property?
		Does it reset all the variables?
		If ''var-foo: initial;'' isn't interpreted by CSS,
		what effect does ''all: initial;'' have?

	A <dfn>custom property</dfn> is any property
	whose name starts with "var-",
	and contains at least one additional character.
	<a>Custom properties</a> 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 <a href="http://www.w3.org/TR/CSS21/syndata.html#escaped-characters">character escaping</a>.
	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.)

	<div class='example'>
		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:

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

		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.
	</div>

	Unlike other CSS properties,
	custom property names are <strong>case-sensitive</strong>.
	The "var-" prefix <strong>must</strong> be written in lower-case.

	<div class='example'>
		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.
	</div>

	The allowed syntax for <a>custom properties</a> is extremely permissive.
	The <dfn>&lt;any-value></dfn> production matches <em>any</em> sequence of one or more tokens,
	so long as the sequence does not contain
	<<bad-string>>, 
	<<bad-url>>, 
	unmatched <<)>>, <<]>>, or <<}>>,
	or top-level <<semicolon>> tokens.

	Note: This definition,
	along with the general CSS syntax rules,
	imply 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.

	Note: While <<any-value>> must represent at least one token,
	that one token may be whitespace.
	This implies that ''var-foo: ;'' is valid,
	and the corresponding ''var(foo)'' call would have a single space as its substitution value,
	but ''var-foo:;'' is invalid.

	<div class='example'>
		For example, the following is a valid custom property:

		<pre>
		var-foo: if(x > 5) this.width = 10;
		</pre>

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

	The values of custom properties,
	and the values of <a>variables</a> substituted into custom properties,
	are <em>case-sensitive</em>,
	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 <a>custom property</a> 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 <a>custom properties</a> is to define <a>cascading variables</a>.
	In CSS, a <dfn title="variable|cascading variable">cascading variable</dfn> 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 <a>custom property</a> defines a corresponding <a>variable</a>
	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: <a>Custom properties</a> 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.

	<div class='example'>
		This style rule:

		<pre>
		:root {
			var-header-color: #06c;
		}
		</pre>

		declares a <a>custom property</a> 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:

		<pre>
		h1 { background-color: var(header-color); }
		</pre>

		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.
	</div>

	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 <code>style</code> attribute,
	can be read or set using the CSSOM, etc..

	<div class='example'>
		If a <a>custom property</a> 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:

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

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

	<a>Custom properties</a> may use variables in their own values to build up composite variables.
	This can create cyclic dependencies
	where two or more <a>custom properties</a> each attempt to use the variable that the other defines;
	doing so makes all the <a>custom properties</a> involved in the cycle
	compute to their initial value (which is a guaranteed-invalid value).

	<div class='example'>
		This example shows a custom property safely using a variable:

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

		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.
	</div>

	<div class='example invalid-example'>
		On the other hand,
		this example shows an invalid instance of variables depending on each other:

		<pre>
		:root {
			var-one: calc(var(two) + 20px);
			var-two: calc(var(one) - 20px);
		}
		</pre>

		Both 'var-one' and 'var-two' now define <a>invalid variables</a> rather than lengths.
	</div>

	It is important to note that
	<a>custom properties</a> resolve any <a>variables</a> in their values at computed-value time,
	which occurs <em title=''>before</em> 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.

	<div class='example'>
		For example,
		given the following structure,
		these custom properties are <strong>not</strong> cyclic,
		and all define valid variables:

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

		The &lt;one> element defines a value for 'var-foo'.
		The &lt;two> element inherits this value,
		and additionally assigns a value to 'var-bar' using the ''foo'' variable.
		Finally,
		the &lt;three> element inherits the 'var-bar' value
		<em>after</em> 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 &lt;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''.
	</div>


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<h2 id='using-variables'>
Using Cascading Variables: the ''var()'' notation</h2>

	Every <a>custom property</a> automatically defines a corresponding <a>cascading variable</a>,
	which can then be substituted into another property with the ''var()'' function.
	The syntax of ''var()'' is:

	<pre class='prod'>
	<dfn id='variable-type' title="<variable>|var()">&lt;variable></dfn> = var( <var>variable-name</var> [, <<any-value>> ]? )
	</pre>

	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 second argument to the function, if provided,
	is a fallback value,
	which is used as the substitution value when the referenced variable is invalid.

	Note: The syntax of the fallback, like that of <a>custom properties</a>, 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.

	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 <a title="resolve a variable">resolved</a>.

	To <dfn>resolve a variable</dfn> in a property's value:

	<ol>
		<li>
			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 <a>custom property</a>.

		<li>
			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,
			<a title="resolve a variable">resolve</a> them as well.

		<li>
			Otherwise,
			the property containing the ''var()'' function is <a>invalid at computed-value time</a>.

			Note: Other things can also make a property <a>invalid at computed-value time</a>.
	</ol>

	<div class='example'>
		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.

		<pre>
		/* 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 */
		}
		</pre>
	</div>

	<div class='example'>
		For example, the following code incorrectly attempts to use a variable as a property name:

		<pre>
		.foo {
			var-side: margin-top;
			var(side): 20px;
		}
		</pre>

		This is <em>not</em> 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:

		<pre>
		.foo {
			var-gap: 20;
			margin-top: var(gap)px;
		}
		</pre>

		Again, this is <em>not</em> 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:

		<pre>
		.foo {
		  var-gap: 20;
		  margin-top: calc(var(gap) * 1px);
		}
		</pre>
	</div>

	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 <a>invalid at computed-value time</a>.

	<div class='example'>
		For example,
		the following usage is fine from a syntax standpoint,
		but results in nonsense when the variable is substituted in:

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

		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.
	</div>


<h3 id='invalid-variables'>
Invalid Variables</h3>

	When a <a>custom property</a> has its initial value,
	the <a>variable</a> it defines
	represents an <dfn>invalid variable</dfn>.
	Using an <a>invalid variable</a> in a property value
	(including other <a>custom properties</a>)
	makes the declaration <a>invalid at computed-value time</a>.

	A declaration can be <dfn>invalid at computed-value time</dfn>
	if it uses an invalid <a>variable</a>, as explained above,
	or if it uses a valid <a>variable</a>,
	but the property value,
	after substituting its <a>variables</a>,
	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.

	<div class='example'>
		For example, in the following code:

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

		the &lt;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.
	</div>

	Note: The <a>invalid at computed-value time</a> 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.

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<h2 id='cssom'>
APIs</h2>

<h3 id='the-cssstyledeclaration-interface'>
Extensions to the <code>CSSStyleDeclaration</code> Interface</h3>

	The <dfn><code>CSSStyleDeclaration</code></dfn> interface is amended as follows:

	<pre class='idl'>
	partial interface CSSStyleDeclaration {
		readonly attribute CSSVariablesMap var;
	}
	</pre>

	A <code>CSSStyleDeclaration</code> is the <dfn>associated style declaration</dfn>
	for the <code>CSSVariablesMap</code> assigned to its <code>var</code> 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.



<h4 id='serializing-custom-props'>
Serializing Custom Properties</h4>

	Custom property names must be serialized with the casing as provided by the author.
	
	<p class='note'>
		Ordinarily, property names are restricted to the ASCII range and are case-insensitive,
		so implementations typically serialize the name lowercased.

<h3 id='the-CSSVariablesMap-interface'>
The <code>CSSVariablesMap</code> Interface</h3>

	The <dfn><code>CSSVariablesMap</code></dfn> interface 
	exposes the <a>custom properties</a> declared in the parent declaration block
	that have a non-initial value.

	<pre class='idl'>
	[MapClass(DOMString, DOMString)]
	interface CSSVariablesMap {
		DOMString get(DOMString varName);
		boolean has(DOMString varName);
		void set(DOMString varName, DOMString varValue);
		boolean delete(DOMString varName);
	}
	</pre>

	The <a href="http://dev.w3.org/2006/webapi/WebIDL/#REPLACE-ME">map tuples</a>
	on a CSSVariablesMap object
	are the property names of all the <a>custom properties</a> in the <a href="http://dev.w3.org/csswg/cssom/#css-declaration-block-declarations"><dfn>CSS declaration block declarations</dfn></a>
	with a non-initial value,
	with the "var-" prefix removed,
	paired with their values.

<h4 id='cssvariablesmap-methods'>
<code>CSSVariablesMap Methods</code></h4>

	<dl>
		<dt>get(DOMString varName), returns DOMString
		<dd>
			Prepend "var-" to <var>varName</var>.
			Invoke <code>getPropertyValue()</code>
			on the <a>associated style declaration</a>
			by passing <var>varName</var> as its argument,
			and return its returned value.

		<dt>has(DOMString varName), returns boolean
		<dd>
			Invoke get() on the CSSVariablesMap object
			by passing <var>varName</var> as its argument.
			If the return value is the empty string,
			return <code>false</code>;
			otherwise, return <code>true</code>.

		<dt>set(DOMString varName, DOMString varValue), returns void
		<dd>
			Prepend "var-" to <var>varName</var>.
			Invoke <code>getPropertyValue()</code>
			on the <a>associated style declaration</a>
			by passing <var>varName</var> and <var>varValue</var> as its two arguments, in that order.

		<dt>delete(DOMString varName), return DOMString
		<dd>
			Prepend "var-" to <var>varName</var>.
			Invoke <code>removeProperty()</code>
			on the <a>associated style declaration</a>
			by passing <var>varName</var> as its argument.
			If the returned value is the empty string,
			return <code>false</code>;
			otherwise, return <code>true</code>.
	</dl>

	<div class='example'>
		<style scoped>
		#var-code-examples td { padding: 2px; }
		#var-code-examples code { background: rgba(0,0,0,.05); display: inline-block; padding: 1px 2px; white-space: pre; }
		</style>
		
		For example,
		given the following style sheet:

		<pre>
		div {
			var-foo: 16px;
			var-Bar: red;
			var-foo-bar: 50%;
		}
		</pre>

		Here are the results of several JavaScript expressions,
		assuming that <code>el</code> is a JavaScript variable
		holding an element that the above style rule applies to:

		<table id="var-code-examples">
			<thead>
				<tr>
					<th>Code
					<th>Value
					<th>Notes
			<tbody>
				<tr>
					<td><code>el.style.var.get("foo")</code>
					<td><code>"16px"</code>
					<td>The value of 'var-foo'.
				<tr>
					<td><code>el.style.var.get("Bar")</code>
					<td><code>"red"</code>
					<td>The value of 'var-Bar'.
				<tr>
					<td><code>el.style.var.get("foo-bar")</code>
					<td><code>"50%"</code>
					<td>The value of 'var-foo-bar'.
				<tr>
					<td><code>el.style.varFoo</code>
					<td>n/a
					<td>Custom properties don't exist directly on "style"
				<tr>
					<td><code>el.style.varBar</code>
					<td>n/a
					<td>Not even if the casing matches.
		</table>
	</div>

	<div class='example'>
		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 <code>var</code> property:

		<pre>
		var customProps = el.style.var;
		for(customPropName in customProps) {
			if( knownCustomPropName(customPropName) ) {
				var customPropValue = customProps[customPropName];
				/* Processing code here. */
			}
		}
		</pre>
	</div>


<h2 id='changes'>
Changes from 10 April 2012 Working Draft</h2>

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


<h2>
Acknowledgments</h2>

	<p>
		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.