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<!DOCTYPE html><html lang=en><head>
<meta content="text/html; charset=utf-8" http-equiv=Content-Type>
<title>CSS Custom Properties for Cascading Variables Module Level 1</title>
<link href=../default.css rel=stylesheet type=text/css>
<link href=../csslogo.ico rel="shortcut icon" type=image/x-icon>
<style>
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padding: 2em 1em 2em 70px;
}
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a[href]:hover { background: #ffa }
a[href] img { border-style: none }
h1, h2, h3, h4, h5, h6 { text-align: left }
h1, h2, h3 { color: #005A9C; }
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p.copyright { font-size: small }
p.copyright small { font-size: small }
pre { margin-left: 2em }
dt { font-weight: bold }
ul.toc, ol.toc {
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</style>
<script defer src=//test.csswg.org/harness/annotate.js#css-variables-1_dev type=text/javascript></script>
</head>
<body class=h-entry>
<div class=head>
<p data-fill-with=logo><a class=logo href=http://www.w3.org/>
<img alt=W3C height=48 src=https://www.w3.org/Icons/w3c_home width=72>
</a>
</p>
<h1 class="p-name no-ref" id=title>CSS Custom Properties for Cascading Variables Module Level 1</h1>
<h2 class="no-num no-toc no-ref heading settled" id=subtitle><span class=content>Editor’s Draft,
<span class=dt-updated><span class=value-title title=20140821>21 August 2014</span></span></span></h2>
<div data-fill-with=spec-metadata><dl>
<dt>This version:
<dd><a class=u-url href=http://dev.w3.org/csswg/css-variables/>http://dev.w3.org/csswg/css-variables/</a>
<dt>Latest version:
<dd><a href=http://www.w3.org/TR/css-variables-1/>http://www.w3.org/TR/css-variables-1/</a>
<dt>Previous Versions:
<dd><dd><a href=http://www.w3.org/TR/2013/WD-css-variables-1-20130620/ rel=previous>http://www.w3.org/TR/2013/WD-css-variables-1-20130620/</a>
<dd><dd><a href=http://www.w3.org/TR/2013/WD-css-variables-20130312/ rel=previous>http://www.w3.org/TR/2013/WD-css-variables-20130312/</a>
<dd><dd><a href=http://www.w3.org/TR/2012/WD-css-variables-20120410/ rel=previous>http://www.w3.org/TR/2012/WD-css-variables-20120410/</a>
<dt>Feedback:
<dd><a href="mailto:www-style@w3.org?subject=%5Bcss-variables%5D%20feedback">www-style@w3.org</a>
with subject line
“<kbd>[css-variables] <var>… message topic …</var></kbd>”(<a href=http://lists.w3.org/Archives/Public/www-style/ rel=discussion>archives</a>)
<dt>Test Suite:
<dd><a href=http://test.csswg.org/suites/css-variables-1_dev/nightly-unstable/>http://test.csswg.org/suites/css-variables-1_dev/nightly-unstable/</a>
<dt>Editor:
<dd><dd class="p-author h-card vcard"><a class="p-name fn u-url url" href=http://xanthir.com/contact>Tab Atkins Jr.</a> (<span class="p-org org">Google</span>)
<dt>Date:
<dd>2014-05-06
</dl>
</div>
<div data-fill-with=warning></div>
<p class=copyright data-fill-with=copyright><a href=http://www.w3.org/Consortium/Legal/ipr-notice#Copyright>Copyright</a> © 2014 <a href=http://www.w3.org/><abbr title="World Wide Web Consortium">W3C</abbr></a><sup>®</sup> (<a href=http://www.csail.mit.edu/><abbr title="Massachusetts Institute of Technology">MIT</abbr></a>, <a href=http://www.ercim.eu/><abbr title="European Research Consortium for Informatics and Mathematics">ERCIM</abbr></a>, <a href=http://www.keio.ac.jp/>Keio</a>, <a href=http://ev.buaa.edu.cn/>Beihang</a>), All Rights Reserved. W3C <a href=http://www.w3.org/Consortium/Legal/ipr-notice#Legal_Disclaimer>liability</a>, <a href=http://www.w3.org/Consortium/Legal/ipr-notice#W3C_Trademarks>trademark</a> and <a href=http://www.w3.org/Consortium/Legal/copyright-documents>document use</a> rules apply.
</p>
<hr title="Separator for header">
</div>
<h2 class="no-num no-toc no-ref heading settled" id=abstract><span class=content>Abstract</span></h2>
<div class=p-summary data-fill-with=abstract><p>This module introduces cascading variables as a new primitive value type that is accepted by all CSS properties, and custom properties for defining them.</p>
<a href=http://www.w3.org/TR/CSS/>CSS</a> is a language for describing the rendering of structured documents
(such as HTML and XML)
on screen, on paper, in speech, etc.</div>
<h2 class="no-num no-toc no-ref heading settled" id=status><span class=content>Status of this document</span></h2>
<div data-fill-with=status><p>
This is a public copy of the editors’ draft.
It is provided for discussion only and may change at any moment.
Its publication here does not imply endorsement of its contents by W3C.
Don’t cite this document other than as work in progress.
<p>
The (<a href=http://lists.w3.org/Archives/Public/www-style/>archived</a>) public mailing list
<a href="mailto:www-style@w3.org?Subject=%5Bcss-variables%5D%20PUT%20SUBJECT%20HERE">www-style@w3.org</a>
(see <a href=http://www.w3.org/Mail/Request>instructions</a>)
is preferred for discussion of this specification.
When sending e-mail,
please put the text “css-variables” in the subject,
preferably like this:
“[css-variables] <em>…summary of comment…</em>”
<p>
This document was produced by the <a href=http://www.w3.org/Style/CSS/members>CSS Working Group</a>
(part of the <a href=http://www.w3.org/Style/>Style Activity</a>).
<p>
This document was produced by a group operating under
the <a href=http://www.w3.org/Consortium/Patent-Policy-20040205/>5 February 2004 W3C Patent Policy</a>.
W3C maintains a <a href=http://www.w3.org/2004/01/pp-impl/32061/status rel=disclosure>public list of any patent disclosures</a>
made in connection with the deliverables of the group;
that page also includes instructions for disclosing a patent.
An individual who has actual knowledge of a patent which the individual believes contains <a href=http://www.w3.org/Consortium/Patent-Policy-20040205/#def-essential>Essential Claim(s)</a>
must disclose the information in accordance with <a href=http://www.w3.org/Consortium/Patent-Policy-20040205/#sec-Disclosure>section 6 of the W3C Patent Policy</a>.</div>
<div data-fill-with=at-risk></div>
<h2 class="no-num no-toc no-ref heading settled" id=contents><span class=content>Table of Contents</span></h2>
<div data-fill-with=table-of-contents><ul class=toc>
<li><a href=#intro><span class=secno>1</span> Introduction</a>
<li><a href=#defining-variables><span class=secno>2</span> Defining Custom Properties: the '--*' family of properties</a>
<ul class=toc>
<li><a href=#syntax><span class=secno>2.1</span> Custom Property Value Syntax</a>
<li><a href=#cycles><span class=secno>2.2</span> Resolving Dependency Cycles</a></ul>
<li><a href=#using-variables><span class=secno>3</span> Using Cascading Variables: the <span class=css data-link-type=maybe title=var()>var()</span> notation</a>
<ul class=toc>
<li><a href=#invalid-variables><span class=secno>3.1</span> Invalid Variables</a>
<li><a href=#variables-in-shorthands><span class=secno>3.2</span> Variables in Shorthand Properties</a></ul>
<li><a href=#apis><span class=secno>4</span> APIs</a>
<ul class=toc>
<li><a href=#serializing-custom-props><span class=secno>4.1</span> Serializing Custom Properties</a></ul>
<li><a href=#changes><span class=secno>5</span> Changes since the June 20 2013 Last Call Working Draft</a>
<li><a href=#acks><span class=secno>6</span> Acknowledgments</a>
<li><a href=#conformance><span class=secno></span> Conformance</a>
<ul class=toc>
<li><a href=#conventions><span class=secno></span> Document conventions</a>
<li><a href=#conformance-classes><span class=secno></span> Conformance classes</a>
<li><a href=#partial><span class=secno></span> Partial implementations</a>
<li><a href=#experimental><span class=secno></span> Experimental implementations</a>
<li><a href=#testing><span class=secno></span> Non-experimental implementations</a></ul>
<li><a href=#references><span class=secno></span> References</a>
<ul class=toc>
<li><a href=#normative><span class=secno></span> Normative References</a>
<li><a href=#informative><span class=secno></span> Informative References</a></ul>
<li><a href=#index><span class=secno></span> Index</a>
<li><a href=#property-index><span class=secno></span> Property index</a></ul></div>
<h2 class="heading settled" data-level=1 id=intro><span class=secno>1 </span><span class=content>
Introduction</span><a class=self-link href=#intro></a></h2>
<p><em>This section is not normative.</em></p>
<p>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.</p>
<p>This module introduces a family of custom author-defined properties known collectively as <a data-link-type=dfn href=#custom-property title="custom properties">custom properties</a>,
which allow an author to assign arbitrary values to a property with an author-chosen name,
and the <a class=css data-link-type=maybe href=#funcdef-var title=var()>var()</a> 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 <a data-link-type=dfn href=#custom-property title="custom property">custom property</a>,
and the change will propagate to all uses of that variable automatically.</p>
<h2 class="heading settled" data-level=2 id=defining-variables><span class=secno>2 </span><span class=content>
Defining Custom Properties: the '--*' family of properties</span><a class=self-link href=#defining-variables></a></h2>
<p>This specification defines an open-ended set of properties called <a data-link-type=dfn href=#custom-property title="custom properties">custom properties</a>,
which, among other things, are used to define the <a data-link-type=dfn href=#substitute-a-var title="substitute a var()">substitution value</a> of <a class=css data-link-type=maybe href=#funcdef-var title=var()>var()</a> functions.</p>
<table class="definition propdef"><tr><th>Name:<td><dfn class=css data-dfn-type=property data-export="" id=propdef--->--*<a class=self-link href=#propdef---></a></dfn><tr><th>Value:<td class=prod><a class="production css" data-link-type=type href=#typedef-any-value title="<any-value>"><any-value></a><tr><th>Initial:<td>(nothing, see prose)<tr><th>Applies to:<td>all elements<tr><th>Inherited:<td>yes<tr><th>Media:<td>all<tr><th>Computed value:<td>specified value with variables substituted (but see prose for "invalid variables")<tr><th>Animatable:<td>no</table>
<p>A <dfn data-dfn-type=dfn data-export="" id=custom-property>custom property<a class=self-link href=#custom-property></a></dfn> is any property
whose name starts with two dashes (U+002D HYPHEN-MINUS),
like <span class=css>--foo</span>.
The <dfn class=css data-dfn-type=type data-export="" id=typedef-custom-property-name><custom-property-name><a class=self-link href=#typedef-custom-property-name></a></dfn> production corresponds to this:
it’s defined as any valid <a data-link-type=dfn href=http://dev.w3.org/csswg/css-syntax-3/#identifier title=identifier>identifier</a> that starts with two dashes.
<a data-link-type=dfn href=#custom-property title="Custom properties">Custom properties</a> are solely for use by authors and users;
CSS will never give them a meaning beyond what is presented here.</p>
<div class=example>
Custom properties define variables,
referenced with the <a class=css data-link-type=maybe href=#funcdef-var title=var()>var()</a> 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 {
--main-color: #06c;
--accent-color: #006;
}
/* The rest of the CSS file */
#foo h1 {
color: var(--main-color);
}
</pre>
<p>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.</p>
</div>
<p>Unlike other CSS properties,
custom property names are <a data-link-type=dfn href=http://dev.w3.org/csswg/css-syntax-3/#case-sensitive title=case-sensitive>case-sensitive</a>.</p>
<div class=example>
While both <span class=css>--foo</span> and <span class=css>--FOO</span> are valid,
they are distinct properties -
using <span class=css>var(--foo)</span> will refer to the first one,
while using <span class=css>var(--FOO)</span> will refer to the second.
</div>
<p>Custom properties are <strong>not</strong> reset by the <a class=property data-link-type=propdesc href=http://dev.w3.org/csswg/css-cascade-3/#propdef-all title=all>all</a> property.
<span class=note>We may define a property in the future that resets all variables.</span></p>
<p>The <a data-link-spec=css-values data-link-type=dfn href=http://dev.w3.org/csswg/css-values-3/#css-wide-keywords title="CSS-wide keywords">CSS-wide keywords</a> can be used in custom properties,
with the same meaning as in any another property.</p>
<p class=note>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.</p>
<p class=note>Note: While this module focuses on the use of <a data-link-type=dfn href=#custom-property title="custom properties">custom properties</a> with the <a class=css data-link-type=maybe href=#funcdef-var title=var()>var()</a> 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 <a data-biblio-type=informative data-link-type=biblio href=#biblio-css-extensions title=biblio-CSS-EXTENSIONS>[CSS-EXTENSIONS]</a>
will expand on these use-cases and make them easier to do.</p>
<h3 class="heading settled" data-level=2.1 id=syntax><span class=secno>2.1 </span><span class=content>
Custom Property Value Syntax</span><a class=self-link href=#syntax></a></h3>
<p>The allowed syntax for <a data-link-type=dfn href=#custom-property title="custom properties">custom properties</a> is extremely permissive.
The <dfn class=css data-dfn-type=type data-export="" id=typedef-any-value><any-value><a class=self-link href=#typedef-any-value></a></dfn> production matches <em>any</em> sequence of one or more tokens,
so long as the sequence does not contain
<a class="production css" data-link-type=type href=http://dev.w3.org/csswg/css-syntax-3/#typedef-bad-string-token title="<bad-string-token>"><bad-string-token></a>,
<a class="production css" data-link-type=type href=http://dev.w3.org/csswg/css-syntax-3/#typedef-bad-url-token title="<bad-url-token>"><bad-url-token></a>,
unmatched <<)-token>>, <<]-token>>, or <<}-token>>,
or top-level <a class="production css" data-link-type=type href=http://dev.w3.org/csswg/css-syntax-3/#typedef-semicolon-token title="<semicolon-token>"><semicolon-token></a> tokens or <a class="production css" data-link-type=type href=http://dev.w3.org/csswg/css-syntax-3/#typedef-delim-token title="<delim-token>"><delim-token></a> tokens with a value of "!".</p>
<p class=note>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.</p>
<p class=note>Note: Custom properties can contain a trailing <span class=css>!important</span>,
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 <span class=css>!important</span> from being used,
as the <span class=css>!important</span> is removed before syntax checking happens.</p>
<p class=note>Note: While <a class="production css" data-link-type=type href=#typedef-any-value title="<any-value>"><any-value></a> must represent at least one token,
that one token may be whitespace.
This implies that <a class=css data-link-type=propdesc title=--foo>--foo: ;</a> is valid,
and the corresponding <span class=css>var(--foo)</span> call would have a single space as its substitution value,
but <span class=css>--foo:;</span> is invalid.</p>
<div class=example>
For example, the following is a valid custom property:
<pre>--foo: if(x > 5) this.width = 10;
</pre>
<p>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.</p>
</div>
<p>The values of custom properties,
and the values of <a class=css data-link-type=maybe href=#funcdef-var title=var()>var()</a> functions 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.)</p>
<p>The initial value of a <a data-link-type=dfn href=#custom-property title="custom property">custom property</a> is an empty value;
that is, nothing at all.
This initial value has a special interaction with the <a class=css data-link-type=maybe href=#funcdef-var title=var()>var()</a> notation,
which is explained in the section defining <a class=css data-link-type=maybe href=#funcdef-var title=var()>var()</a>.</p>
<p>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 <a class=css data-link-type=maybe href=http://dev.w3.org/csswg/css-conditional-3/#at-ruledef-media title=@media>@media</a> and other conditional rules,
can be used in HTML’s <code>style</code> attribute,
can be read or set using the CSSOM, etc.</p>
<p>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 <a data-link-type=dfn href=#custom-property title="custom property">custom property</a> used in a <a class=css data-link-type=maybe href=http://dev.w3.org/csswg/css-animations-1/#at-ruledef-keyframes title=@keyframes>@keyframes</a> rule
becomes <dfn data-dfn-type=dfn data-noexport="" id=animation-tainted>animation-tainted<a class=self-link href=#animation-tainted></a></dfn>,
which affects how it is treated when referred to via the <a class=css data-link-type=maybe href=#funcdef-var title=var()>var()</a> function in an animation property.</p>
<div class=example>
This style rule:
<pre>:root {
--header-color: #06c;
}
</pre>
<p>declares a <a data-link-type=dfn href=#custom-property title="custom property">custom property</a> 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 <a class=css data-link-type=maybe href=#funcdef-var title=var()>var()</a> function:</p>
<pre>h1 { background-color: var(--header-color); }
</pre>
<p>The preceding rule is equivalent to writing <a class=css data-link-type=propdesc href=http://dev.w3.org/csswg/css-backgrounds-3/#background-color title=background-color>background-color: #06c;</a>,
except that the variable name makes the origin of the color clearer,
and if <span class=css>var(--header-color)</span> 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.</p>
</div>
<div class=example>
If a <a data-link-type=dfn href=#custom-property title="custom property">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 { --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>
</pre>
</div>
<h3 class="heading settled" data-level=2.2 id=cycles><span class=secno>2.2 </span><span class=content>
Resolving Dependency Cycles</span><a class=self-link href=#cycles></a></h3>
<p><a data-link-type=dfn href=#custom-property title="Custom properties">Custom properties</a> are left almost entirely unevaluated,
except that they allow and evaluate the <a class=css data-link-type=maybe href=#funcdef-var title=var()>var()</a> function in their value.
This can create cyclic dependencies
where a custom property uses a <a class=css data-link-type=maybe href=#funcdef-var title=var()>var()</a> referring to itself,
or two or more <a data-link-type=dfn href=#custom-property title="custom properties">custom properties</a> each attempt to refer to each other.</p>
<p>For each element,
create an undirected dependency graph,
containing nodes for each <a data-link-type=dfn href=#custom-property title="custom property">custom property</a>.
If the value of a <a data-link-type=dfn href=#custom-property title="custom property">custom property</a> <var>prop</var>
contains a <a class=css data-link-type=maybe href=#funcdef-var title=var()>var()</a> function referring to the property <var>var</var>
(including in the fallback argument of <a class=css data-link-type=maybe href=#funcdef-var title=var()>var()</a>),
add an edge between <var>prop</var> and the <var>var</var>.
<span class=note>Edges are possible from a custom property to itself.</span>
If there is a cycle in the dependency graph,
all the <a data-link-type=dfn href=#custom-property title="custom properties">custom properties</a> in the cycle
must compute to their initial value
(which is a guaranteed-invalid value).</p>
<div class=example>
This example shows a custom property safely using a variable:
<pre>:root {
--main-color: #c06;
--accent-background: linear-gradient(to top, var(--main-color), white);
}
</pre>
<p>The '--accent-background' property
(along with any other properties that use <span class=css>var(--main-color)</span>)
will automatically update when the '--main-color' property is changed.</p>
</div>
<div class="example invalid-example">
On the other hand,
this example shows an invalid instance of variables depending on each other:
<pre>:root {
--one: calc(var(--two) + 20px);
--two: calc(var(--one) - 20px);
}
</pre>
<p>Both '--one' and '--two' now compute to their initial value,
rather than lengths.</p>
</div>
<p>It is important to note that
<a data-link-type=dfn href=#custom-property title="custom properties">custom properties</a> resolve any <a class=css data-link-type=maybe href=#funcdef-var title=var()>var()</a> functions in their values at computed-value time,
which occurs <em>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.</p>
<div class=example>
For example,
given the following structure,
these custom properties are <strong>not</strong> cyclic,
and all define valid variables:
<pre><one><two><three /></two></one>
one { --foo: 10px; }
two { --bar: calc(var(--foo) + 10px); }
three { --foo: calc(var(--bar) + 10px); }
</pre>
<p>The <one> element defines a value for '--foo'.
The <two> element inherits this value,
and additionally assigns a value to '--bar' using the <span class=css>foo</span> variable.
Finally,
the <three> element inherits the '--bar' value
<em>after</em> variable substitution
(in other words, it sees the value <span class=css>calc(10px + 10px)</span>),
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 <span class=css>var(--bar)</span> variable is not cyclic,
and actually defines a value that will eventually
(when referenced as a variable in a normal property)
resolve to <span class=css>30px</span>.</p>
</div>
<h2 class="heading settled" data-level=3 id=using-variables><span class=secno>3 </span><span class=content>
Using Cascading Variables: the <a class=css data-link-type=maybe href=#funcdef-var title=var()>var()</a> notation</span><a class=self-link href=#using-variables></a></h2>
<p>The value of a <a data-link-type=dfn href=#custom-property title="custom property">custom property</a> can be substituted into the value of another property
with the <a class=css data-link-type=maybe href=#funcdef-var title=var()>var()</a> function.
The syntax of <a class=css data-link-type=maybe href=#funcdef-var title=var()>var()</a> is:</p>
<pre class=prod><dfn class=css data-dfn-type=function data-export="" id=funcdef-var>var()<a class=self-link href=#funcdef-var></a></dfn> = var( <a class="production css" data-link-type=type href=#typedef-custom-property-name title="<custom-property-name>"><custom-property-name></a> [<a class=prod-punc data-link-type=grammar href=http://dev.w3.org/csswg/css-values-3/#comb-comma title=,>,</a> <a class="production css" data-link-type=type href=#typedef-any-value title="<any-value>"><any-value></a> ]<a class=prod-punc data-link-type=grammar href=http://dev.w3.org/csswg/css-values-3/#mult-opt title=?>?</a> )
</pre>
<p>The <a class=css data-link-type=maybe href=#funcdef-var title=var()>var()</a> function can be used in place of any part of a value in any property on an element.
The <a class=css data-link-type=maybe href=#funcdef-var title=var()>var()</a> 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.)</p>
<p>The first argument to the function is the name of the <a data-link-type=dfn href=#custom-property title="custom property">custom property</a> 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 <a data-link-type=dfn href=#custom-property title="custom property">custom property</a> is invalid.</p>
<p class=note>Note: The syntax of the fallback, like that of <a data-link-type=dfn href=#custom-property title="custom properties">custom properties</a>, allows commas.
For example, <span class=css>var(--foo, red, blue)</span> defines a fallback of <span class=css>red, blue</span>;
that is, anything between the first comma and the end of the function is considered a fallback value.</p>
<p>If a property contains one or more <a class=css data-link-type=maybe href=#funcdef-var title=var()>var()</a> 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 <a class=css data-link-type=maybe href=#funcdef-var title=var()>var()</a> functions have been <a data-link-type=dfn href=#substitute-a-var title="substitute a var()">substituted</a>.</p>
<p>To <dfn data-dfn-type=dfn data-export="" id=substitute-a-var>substitute a var()<a class=self-link href=#substitute-a-var></a></dfn> in a property’s value:</p>
<ol>
<li>
If the <a data-link-type=dfn href=#custom-property title="custom property">custom property</a> named by the first argument to the <a class=css data-link-type=maybe href=#funcdef-var title=var()>var()</a> function is <a data-link-type=dfn href=#animation-tainted title=animation-tainted>animation-tainted</a>,
and the <a class=css data-link-type=maybe href=#funcdef-var title=var()>var()</a> function is being used in the <a class=property data-link-type=propdesc href=http://dev.w3.org/csswg/css-animations-1/#propdef-animation title=animation>animation</a> property or one of its longhands,
treat the <a data-link-type=dfn href=#custom-property title="custom property">custom property</a> as having its initial value for the rest of this algorithm.
<li>
If the value of the <a data-link-type=dfn href=#custom-property title="custom property">custom property</a> named by the first argument to the <a class=css data-link-type=maybe href=#funcdef-var title=var()>var()</a> function
is anything but the initial value,
replace the <a class=css data-link-type=maybe href=#funcdef-var title=var()>var()</a> function by the value of the corresponding <a data-link-type=dfn href=#custom-property title="custom property">custom property</a>.
<li>
Otherwise, if the <a class=css data-link-type=maybe href=#funcdef-var title=var()>var()</a> function has a fallback value as its second argument,
replace the <a class=css data-link-type=maybe href=#funcdef-var title=var()>var()</a> function by the fallback value.
If there are any <a class=css data-link-type=maybe href=#funcdef-var title=var()>var()</a> references in the fallback,
<a data-link-type=dfn href=#substitute-a-var title="substitute a var()">substitute</a> them as well.
<li>
Otherwise,
the property containing the <a class=css data-link-type=maybe href=#funcdef-var title=var()>var()</a> function is <a data-link-type=dfn href=#invalid-at-computed-value-time title="invalid at computed-value time">invalid at computed-value time</a>.
<p class=note>Note: Other things can also make a property <a data-link-type=dfn href=#invalid-at-computed-value-time title="invalid at computed-value time">invalid at computed-value time</a>.</p>
</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.
<p>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.</p>
<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 {
--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 {
--side: margin-top;
var(--side): 20px;
}
</pre>
<p>This is <em>not</em> equivalent to setting <a class=css data-link-type=propdesc href=http://dev.w3.org/csswg/css2/box.html#propdef-margin-top title=margin-top>margin-top: 20px;</a>.
Instead, the second declaration is simply thrown away as a syntax error
for having an invalid property name.</p>
<p>Similarly, you can’t build up a single token where part of it is provided by a variable:</p>
<pre>.foo {
--gap: 20;
margin-top: var(--gap)px;
}
</pre>
<p>Again, this is <em>not</em> equivalent to setting <a class=css data-link-type=propdesc href=http://dev.w3.org/csswg/css2/box.html#propdef-margin-top title=margin-top>margin-top: 20px;</a> (a length).
Instead, it’s equivalent to <a class=css data-link-type=propdesc href=http://dev.w3.org/csswg/css2/box.html#propdef-margin-top title=margin-top>margin-top: 20 px;</a> (a number followed by an ident),
which is simply an invalid value for the <a class=property data-link-type=propdesc href=http://dev.w3.org/csswg/css2/box.html#propdef-margin-top title=margin-top>margin-top</a> property.
Note, though, that <a class=css data-link-type=maybe href=http://dev.w3.org/csswg/css-values-3/#funcdef-calc title=calc()>calc()</a> can be used to validly achieve the same thing, like so:</p>
<pre>.foo {
--gap: 20;
margin-top: calc(var(--gap) * 1px);
}
</pre>
</div>
<p><a class=css data-link-type=maybe href=#funcdef-var title=var()>var()</a> functions are <a data-link-type=dfn href=#substitute-a-var title="substitute a var()">substituted</a> at computed-value time.
If a declaration,
once all <a class=css data-link-type=maybe href=#funcdef-var title=var()>var()</a> functions are substituted in,
is invalid,
the declaration is <a data-link-type=dfn href=#invalid-at-computed-value-time title="invalid at computed-value time">invalid at computed-value time</a>.</p>
<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 { --looks-valid: 20px; }
p { background-color: var(--looks-valid); }
</pre>
<p>Since <span class=css>20px</span> is an invalid value for <a class=property data-link-type=propdesc href=http://dev.w3.org/csswg/css-backgrounds-3/#background-color title=background-color>background-color</a>,
this instance of the property computes to <span class=css>transparent</span>
(the initial value for <a class=property data-link-type=propdesc href=http://dev.w3.org/csswg/css-backgrounds-3/#background-color title=background-color>background-color</a>)
instead.</p>
<p>If the property was one that’s inherited by default,
such as <a class=property data-link-type=propdesc href=http://dev.w3.org/csswg/css-color-3/#color0 title=color>color</a>,
it would compute to the inherited value
rather than the initial value.</p>
</div>
<h3 class="heading settled" data-level=3.1 id=invalid-variables><span class=secno>3.1 </span><span class=content>
Invalid Variables</span><a class=self-link href=#invalid-variables></a></h3>
<p>When a <a data-link-type=dfn href=#custom-property title="custom property">custom property</a> has its initial value,
<a class=css data-link-type=maybe href=#funcdef-var title=var()>var()</a> functions cannot use it for substitution.
Attempting to do so
makes the declaration <a data-link-type=dfn href=#invalid-at-computed-value-time title="invalid at computed-value time">invalid at computed-value time</a>,
unless a valid fallback is specified.</p>
<p>A declaration can be <dfn data-dfn-type=dfn data-noexport="" id=invalid-at-computed-value-time>invalid at computed-value time<a class=self-link href=#invalid-at-computed-value-time></a></dfn>
if it contains a <a class=css data-link-type=maybe href=#funcdef-var title=var()>var()</a> that references a <a data-link-type=dfn href=#custom-property title="custom property">custom property</a> with its initial value,
as explained above,
or if it uses a valid <a data-link-type=dfn href=#custom-property title="custom property">custom property</a>,
but the property value,
after substituting its <a class=css data-link-type=maybe href=#funcdef-var title=var()>var()</a> 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 <span class=css>unset</span> keyword.</p>
<div class=example>
For example, in the following code:
<pre>:root { --not-a-color: 20px; }
p { background-color: red; }
p { background-color: var(--not-a-color); }
</pre>
<p>the <p> elements will have transparent backgrounds
(the initial value for <a class=property data-link-type=propdesc href=http://dev.w3.org/csswg/css-backgrounds-3/#background-color title=background-color>background-color</a>),
rather than red backgrounds.
The same would happen if the <a data-link-type=dfn href=#custom-property title="custom property">custom property</a> itself was unset,
or contained an invalid <a class=css data-link-type=maybe href=#funcdef-var title=var()>var()</a> function.</p>
<p>Note the difference between this
and what happens if the author had just written <a class=css data-link-type=propdesc href=http://dev.w3.org/csswg/css-backgrounds-3/#background-color title=background-color>background-color: 20px</a> directly in their stylesheet -
that would be a normal syntax error,
which would cause the rule to be discarded,
so the <a class=css data-link-type=propdesc href=http://dev.w3.org/csswg/css-backgrounds-3/#background-color title=background-color>background-color: red</a> rule would be used instead.</p>
</div>
<p class=note>Note: The <a data-link-type=dfn href=#invalid-at-computed-value-time title="invalid at computed-value time">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.</p>
<h3 class="heading settled" data-level=3.2 id=variables-in-shorthands><span class=secno>3.2 </span><span class=content>
Variables in Shorthand Properties</span><a class=self-link href=#variables-in-shorthands></a></h3>
<p>The use of <a class=css data-link-type=maybe href=#funcdef-var title=var()>var()</a> functions in <a data-link-type=dfn href=http://dev.w3.org/csswg/css-cascade-3/#shorthand-property title="shorthand properties">shorthand properties</a> presents some unique difficulties.</p>
<p>Ordinarily, the value of a shorthand property is separated out into its component <a data-link-type=dfn href=http://dev.w3.org/csswg/css-cascade-3/#longhand title="longhand properties">longhand properties</a> at parse time,
and then the longhands themselves participate in the <a data-link-type=dfn href=http://dev.w3.org/csswg/css-cascade-3/#cascade title=cascade>cascade</a>,
with the shorthand more-or-less discarded.
If a <a class=css data-link-type=maybe href=#funcdef-var title=var()>var()</a> 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 <a class=css data-link-type=maybe href=#funcdef-var title=var()>var()</a> function may substitute in the value of several longhands at once.</p>
<p>To get around this,
implementations must fill in longhands with a special, unobservable-to-authors <dfn data-dfn-type=dfn data-export="" id=pending-substitution-value>pending-substitution value<a class=self-link href=#pending-substitution-value></a></dfn>
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 <a class=css data-link-type=maybe href=#funcdef-var title=var()>var()</a> functions are finally substituted in,
the shorthand must be parsed and the longhands must be given their appropriate values at that point.</p>
<p><a data-link-type=dfn href=#pending-substitution-value title="Pending-substitution values">Pending-substitution values</a> must be serialized as the empty string,
if an API allows them to be observed.</p>
<p>Similarly,
while <a data-biblio-type=informative data-link-type=biblio href=#biblio-cssom title=biblio-CSSOM>[CSSOM]</a> defines that shorthand properties are serialized
by appropriately concatenating the values of their corresponding longhands,
shorthands that are specified with explicit <a class=css data-link-type=maybe href=#funcdef-var title=var()>var()</a> functions
must serialize to the original, <a class=css data-link-type=maybe href=#funcdef-var title=var()>var()</a>-containing value.
For other shorthands,
if any of the longhand subproperties for that shorthand have <a data-link-type=dfn href=#pending-substitution-value title="pending-substitution values">pending-substitution values</a>
then the serialized value of the shorthand must be the empty string.</p>
<h2 class="heading settled" data-level=4 id=apis><span class=secno>4 </span><span class=content>
APIs</span><a class=self-link href=#apis></a></h2>
<p>All <a data-link-type=dfn href=#custom-property title="custom property">custom property</a> <a data-link-type=dfn href=http://dev.w3.org/csswg/css-syntax-3/#declaration title=declarations>declarations</a> have the <a data-link-type=dfn title=case-insensitive>case-insensitive</a> flag set.</p>
<p class=note>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 <a class=idl-code data-link-type=method title=getPropertyValue()>getPropertyValue()</a>/etc.</p>
<h3 class="heading settled" data-level=4.1 id=serializing-custom-props><span class=secno>4.1 </span><span class=content>
Serializing Custom Properties</span><a class=self-link href=#serializing-custom-props></a></h3>
<p>Custom property names must be serialized with the casing as provided by the author.</p>
<p class=note>
Ordinarily, property names are restricted to the ASCII range and are case-insensitive,
so implementations typically serialize the name lowercased.
<h2 class="heading settled" data-level=5 id=changes><span class=secno>5 </span><span class=content>
Changes since the June 20 2013 Last Call Working Draft</span><a class=self-link href=#changes></a></h2>
<ul>
<li>
The syntax of custom properties was changed from <span class=css>var-foo</span> to <span class=css>--foo</span>.
<li>
The <a class=css data-link-type=maybe href=#funcdef-var title=var()>var()</a> function takes the full custom property name as its first argument,
rather than the name minus a prefix.
<li>
The CSSVariablesMap interface was removed,
in favor of a better custom-property API to be defined in the future.
</ul>
<h2 class="heading settled" data-level=6 id=acks><span class=secno>6 </span><span class=content>
Acknowledgments</span><a class=self-link href=#acks></a></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.
<h2 class="no-ref no-num heading settled" id=conformance><span class=content>
Conformance</span><a class=self-link href=#conformance></a></h2>
<h3 class="no-ref heading settled" id=conventions><span class=content>
Document conventions</span><a class=self-link href=#conventions></a></h3>
<p>Conformance requirements are expressed with a combination of
descriptive assertions and RFC 2119 terminology. The key words "MUST",
"MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT",
"RECOMMENDED", "MAY", and "OPTIONAL" in the normative parts of this
document are to be interpreted as described in RFC 2119.
However, for readability, these words do not appear in all uppercase
letters in this specification.
<p>All of the text of this specification is normative except sections
explicitly marked as non-normative, examples, and notes. <a data-biblio-type=normative data-link-type=biblio href=#biblio-rfc2119 title=biblio-RFC2119>[RFC2119]</a></p>
<p>Examples in this specification are introduced with the words "for example"
or are set apart from the normative text with <code>class="example"</code>,
like this:
<div class=example>
<p>This is an example of an informative example.</p>
</div>
<p>Informative notes begin with the word "Note" and are set apart from the
normative text with <code>class="note"</code>, like this:
<p class=note>Note, this is an informative note.</p>
<p>Advisements are normative sections styled to evoke special attention and are
set apart from other normative text with <code><strong class="advisement"></code>, like
this:
<strong class=advisement>
UAs MUST provide an accessible alternative.
</strong>
<h3 class="no-ref heading settled" id=conformance-classes><span class=content>
Conformance classes</span><a class=self-link href=#conformance-classes></a></h3>
<p>Conformance to this specification
is defined for three conformance classes:
<dl>
<dt>style sheet
<dd>A <a href=http://www.w3.org/TR/CSS21/conform.html#style-sheet>CSS
style sheet</a>.
<dt>renderer
<dd>A <a href=http://www.w3.org/TR/CSS21/conform.html#user-agent>UA</a>
that interprets the semantics of a style sheet and renders
documents that use them.
<dt>authoring tool
<dd>A <a href=http://www.w3.org/TR/CSS21/conform.html#user-agent>UA</a>
that writes a style sheet.
</dl>
<p>A style sheet is conformant to this specification
if all of its statements that use syntax defined in this module are valid
according to the generic CSS grammar and the individual grammars of each
feature defined in this module.
<p>A renderer is conformant to this specification
if, in addition to interpreting the style sheet as defined by the
appropriate specifications, it supports all the features defined
by this specification by parsing them correctly
and rendering the document accordingly. However, the inability of a
UA to correctly render a document due to limitations of the device
does not make the UA non-conformant. (For example, a UA is not
required to render color on a monochrome monitor.)
<p>An authoring tool is conformant to this specification
if it writes style sheets that are syntactically correct according to the
generic CSS grammar and the individual grammars of each feature in
this module, and meet all other conformance requirements of style sheets
as described in this module.
<h3 class="no-ref heading settled" id=partial><span class=content>
Partial implementations</span><a class=self-link href=#partial></a></h3>
<p>So that authors can exploit the forward-compatible parsing rules to
assign fallback values, CSS renderers <strong>must</strong>
treat as invalid (and <a href=http://www.w3.org/TR/CSS21/conform.html#ignore>ignore
as appropriate</a>) any at-rules, properties, property values, keywords,
and other syntactic constructs for which they have no usable level of
support. In particular, user agents <strong>must not</strong> selectively
ignore unsupported component values and honor supported values in a single
multi-value property declaration: if any value is considered invalid
(as unsupported values must be), CSS requires that the entire declaration
be ignored.</p>
<h3 class="no-ref heading settled" id=experimental><span class=content>
Experimental implementations</span><a class=self-link href=#experimental></a></h3>
<p>To avoid clashes with future CSS features, the CSS2.1 specification
reserves a <a href=http://www.w3.org/TR/CSS21/syndata.html#vendor-keywords>prefixed
syntax</a> for proprietary and experimental extensions to CSS.
<p>Prior to a specification reaching the Candidate Recommendation stage
in the W3C process, all implementations of a CSS feature are considered
experimental. The CSS Working Group recommends that implementations
use a vendor-prefixed syntax for such features, including those in
W3C Working Drafts. This avoids incompatibilities with future changes
in the draft.
</p>
<h3 class="no-ref heading settled" id=testing><span class=content>
Non-experimental implementations</span><a class=self-link href=#testing></a></h3>
<p>Once a specification reaches the Candidate Recommendation stage,
non-experimental implementations are possible, and implementors should
release an unprefixed implementation of any CR-level feature they
can demonstrate to be correctly implemented according to spec.
<p>To establish and maintain the interoperability of CSS across
implementations, the CSS Working Group requests that non-experimental
CSS renderers submit an implementation report (and, if necessary, the
testcases used for that implementation report) to the W3C before
releasing an unprefixed implementation of any CSS features. Testcases
submitted to W3C are subject to review and correction by the CSS
Working Group.
<p>Further information on submitting testcases and implementation reports
can be found from on the CSS Working Group’s website at
<a href=http://www.w3.org/Style/CSS/Test/>http://www.w3.org/Style/CSS/Test/</a>.
Questions should be directed to the
<a href=http://lists.w3.org/Archives/Public/public-css-testsuite>public-css-testsuite@w3.org</a>
mailing list.
<h2 class="no-num no-ref heading settled" id=references><span class=content>
References</span><a class=self-link href=#references></a></h2>
<h3 class="no-num no-ref heading settled" id=normative><span class=content>
Normative References</span><a class=self-link href=#normative></a></h3>
<div data-fill-with=normative-references><dl>
<dt id=biblio-rfc2119 title=RFC2119><a class=self-link href=#biblio-rfc2119></a>[RFC2119]<dd>S. Bradner. <a href=http://www.ietf.org/rfc/rfc2119.txt>Key words for use in RFCs to Indicate Requirement Levels</a>. URL: <a href=http://www.ietf.org/rfc/rfc2119.txt>http://www.ietf.org/rfc/rfc2119.txt</a></dd>
</dl></div>
<h3 class="no-num no-ref heading settled" id=informative><span class=content>
Informative References</span><a class=self-link href=#informative></a></h3>
<div data-fill-with=informative-references><dl>
<dt id=biblio-css-extensions title=CSS-EXTENSIONS><a class=self-link href=#biblio-css-extensions></a>[CSS-EXTENSIONS]<dd>Tab Atkins Jr.. <a href=http://dev.w3.org/csswg/css-extensions/>CSS Extensions</a>. W3C Editor's Draft. (Work in progress.) URL: <a href=http://dev.w3.org/csswg/css-extensions/>http://dev.w3.org/csswg/css-extensions/</a></dd>
<dt id=biblio-cssom title=CSSOM><a class=self-link href=#biblio-cssom></a>[CSSOM]<dd>Anne van Kesteren. <a href=http://www.w3.org/TR/2011/WD-cssom-20110712/>CSSOM</a>. 12 July 2011. W3C Working Draft. (Work in progress.) URL: <a href=http://www.w3.org/TR/2011/WD-cssom-20110712/>http://www.w3.org/TR/2011/WD-cssom-20110712/</a></dd>
</dl></div>
<h2 class="no-num no-ref heading settled" id=index><span class=content>
Index</span><a class=self-link href=#index></a></h2>
<div data-fill-with=index><ul class=indexlist>
<li>--*, <a href=#propdef--- title="section 2">2</a>
<li>animation-tainted, <a href=#animation-tainted title="section 2.1">2.1</a>
<li><any-value>, <a href=#typedef-any-value title="section 2.1">2.1</a>
<li>custom property, <a href=#custom-property title="section 2">2</a>
<li><custom-property-name>, <a href=#typedef-custom-property-name title="section 2">2</a>
<li>invalid at computed-value time, <a href=#invalid-at-computed-value-time title="section 3.1">3.1</a>
<li>pending-substitution value, <a href=#pending-substitution-value title="section 3.2">3.2</a>
<li>substitute a var(), <a href=#substitute-a-var title="section 3">3</a>
<li>var(), <a href=#funcdef-var title="section 3">3</a>
</ul></div>
<h2 class="no-num no-ref heading settled" id=property-index><span class=content>
Property index</span><a class=self-link href=#property-index></a></h2>
<div data-fill-with=property-index><table class="proptable data"><thead><tr><th scope=col>Name<th scope=col>Value<th scope=col>Initial<th scope=col>Applies to<th scope=col>Inh.<th scope=col>%ages<th scope=col>Media<th scope=col>Animatable<th scope=col>Computed value<tbody>
<tr><th scope=row><a class=css data-link-type=property title=--*>--*</a><td><any-value><td>(nothing, see prose)<td>all elements<td>yes<td><td>all<td>no<td>specified value with variables substituted (but see prose for "invalid variables")</table></div>