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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>
<link href=https://www.w3.org/StyleSheets/TR/W3C-ED rel=stylesheet type=text/css>
</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=http://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" id=subtitle>Editor's Draft,
<span class=dt-updated><span class=value-title title=20130627>27 June 2013</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>Editor's Draft:<dd><a href=http://dev.w3.org/csswg/css-variables/>http://dev.w3.org/csswg/css-variables/</a><dt>Previous Versions:<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><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:</dt>
<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>Editors:
<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>)</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> © 2013 <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" id=abstract>Abstract</h2>
<p class=p-summary data-fill-with=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.
<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.</p>
<h2 class="no-num no-toc no-ref" id=status>Status of this document</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=/Style/CSS/members>CSS Working Group</a>
(part of the <a href=/Style/>Style Activity</a>).
<p>
This document was produced by a group operating under
the <a href=/Consortium/Patent-Policy-20040205/>5 February 2004 W3C Patent Policy</a>.
W3C maintains a <a href=/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=/Consortium/Patent-Policy-20040205/#def-essential>Essential Claim(s)</a>
must disclose the information in accordance with <a href=/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" id=contents>Table of contents</h2>
<div data-fill-with=table-of-contents><ul class=toc><li><a href=#introduction><span class=secno>1 </span>
Introduction</a><ul class=toc><li><a href=#module-interactions><span class=secno>1.1 </span>
Module Interactions</a><li><a href=#values><span class=secno>1.2 </span>
Values</a></ul><li><a href=#defining-variables><span class=secno>2 </span>
Defining Custom Properties: the <span class=property data-autolink=property href=#var- title=var-*>var-*</span> 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=#using><span class=secno>2.2 </span>
Using Custom Properties</a><li><a href=#cycles><span class=secno>2.3 </span>
Resolving Dependency Cycles</a></ul><li><a href=#using-variables><span class=secno>3 </span>
Using Cascading Variables: the <span class=css data-autolink=maybe href=#variable-type>var()</span> notation</a><ul class=toc><li><a href=#invalid-variables><span class=secno>3.1 </span>
Invalid Variables</a></ul><li><a href=#cssom><span class=secno>4 </span>
APIs</a><ul class=toc><li><a href=#the-cssstyledeclaration-interface><span class=secno>4.1 </span>
Extensions to the <code>CSSStyleDeclaration</code> Interface</a><ul class=toc><li><a href=#serializing-custom-props><span class=secno>4.1.1 </span>
Serializing Custom Properties</a></ul><li><a href=#the-CSSVariablesMap-interface><span class=secno>4.2 </span>
The <code>CSSVariablesMap</code> Interface</a><ul class=toc><li><a href=#cssvariablesmap-methods><span class=secno>4.2.1 </span>
<code>CSSVariablesMap Methods</code></a></ul></ul><li><a href=#changes><span class=secno>5 </span>
Changes from 10 April 2012 Working Draft</a><li><a href=#acknowledgments><span class=secno>6 </span>
Acknowledgments</a><li><a href=#conformance>
Conformance</a><ul class=toc><li><a href=#conventions>
Document conventions</a><li><a href=#conformance-classes>
Conformance classes</a><li><a href=#partial>
Partial implementations</a><li><a href=#experimental>
Experimental implementations</a><li><a href=#testing>
Non-experimental implementations</a></ul><li><a href=#references>
References</a><ul class=toc><li><a href=#normative>
Normative References</a><li><a href=#informative>
Informative References</a></ul><li><a href=#index>
Index</a><li><a href=#property-index>
Property index</a></ul></div>
<h2 data-level=1 id=introduction><span class=secno>1 </span>
Introduction</h2>
<p> <em>This section is not normative.</em>
<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> This module introduces a family of custom author-defined properties known collectively as <a data-autolink=link href=#custom-property>custom properties</a>,
which allow an author to assign arbitrary values to a property with an author-chosen name,
and <a data-autolink=link href=#cascading-variable>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 data-autolink=link href=#custom-property>custom property</a>,
and the change will propagate to all uses of that variable automatically.
<h3 data-level=1.1 id=module-interactions><span class=secno>1.1 </span>
Module Interactions</h3>
<p> This module defines a new type of primitive value,
the <a data-autolink=link href=#cascading-variable>variable</a>,
which is accepted by all properties.
<h3 data-level=1.2 id=values><span class=secno>1.2 </span>
Values</h3>
<p> This specification follows the <a href=http://www.w3.org/TR/CSS21/about.html#property-defs>CSS property definition conventions</a> from <a data-autolink=biblio data-biblio-type=normative href=#css21 title=CSS21>[CSS21]</a>.
Value types not defined in this specification are defined in CSS Level 2 Revision 1 <a data-autolink=biblio data-biblio-type=normative href=#css21 title=CSS21>[CSS21]</a>.
Other CSS modules may expand the definitions of these value types:
for example if the <a class=production data-autolink=link><var><color></var></a> type was used in this specification,
then <a data-autolink=biblio data-biblio-type=informative href=#css3color title=CSS3COLOR>[CSS3COLOR]</a> would expand its definition.
<h2 data-level=2 id=defining-variables><span class=secno>2 </span>
Defining Custom Properties: the <a class=property data-autolink=property href=#var- title=var-*>var-*</a> family of properties</h2>
<p> This specification defines an open-ended set of properties called <a data-autolink=link href=#custom-property>custom properties</a>,
which are used to define <a data-autolink=link href=#cascading-variable>variables</a>.
<table class=propdef><tr><th>Name:<td><dfn id=var->var-*</dfn><tr><th>Value:<td><a class=production data-autolink=link href=#any-value-production><var><any-value></var></a><tr><th>Initial:<td>(nothing, see prose)<tr><th>Applies to:<td>all elements<tr><th>Inherited:<td>yes<tr><th>Computed value:<td>specified value with variables substituted (but see prose for "invalid variables")<tr><th>Media:<td>all<tr><th>Animatable:<td>no</table>
<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 <span class=css data-autolink=maybe>var-foo: var(foo);</span>,
but you can't get <span class=css data-autolink=maybe>default</span> behavior.
<p class=issue>
What's the relationship of custom properties and the <a class=property data-autolink=property title=all>all</a> property?
Does it reset all the variables?
If <span class=css data-autolink=maybe>var-foo: initial;</span> isn't interpreted by CSS,
what effect does <span class=css data-autolink=maybe>all: initial;</span> have?
<p> A <dfn id=custom-property>custom property</dfn> is any property
whose name starts with "var-",
and contains at least one additional character.
<a data-autolink=link href=#custom-property>Custom properties</a> are solely for use by authors and users;
CSS will never give them a meaning beyond what is presented here.
<p class=note> 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, <a class=property data-autolink=property title=var-0>var-0</a> needs to be referenced as <span class=css data-autolink=maybe>var(\30)</span>,
because <span class=css data-autolink=maybe>0</span> isn't a valid identifier.
(U+0030 is the hexadecimal code for the <span class=css data-autolink=maybe>0</span> character.)
<div class=example>
Custom properties define variables,
referenced with the <a class=css data-autolink=maybe href=#variable-type>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 {
var-main-color: #06c;
var-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.
</div>
<p> 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, <span class=css data-autolink=maybe>VAR-FOO</span> is invalid,
because the prefix isn't "var-".
<p> While both <span class=css data-autolink=maybe>var-foo</span> and <span class=css data-autolink=maybe>var-FOO</span> are valid,
they are distinct properties -
using <span class=css data-autolink=maybe>var(foo)</span> will refer to the first one,
while using <span class=css data-autolink=maybe>var(FOO)</span> will refer to the second.
</div>
<h3 data-level=2.1 id=syntax><span class=secno>2.1 </span>
Custom Property Value Syntax</h3>
<p> The allowed syntax for <a data-autolink=link href=#custom-property>custom properties</a> is extremely permissive.
The <dfn id=any-value-production><any-value></dfn> production matches <em>any</em> sequence of one or more tokens,
so long as the sequence does not contain
<a class=production data-autolink=link><var><bad-string></var></a>,
<a class=production data-autolink=link><var><bad-url></var></a>,
unmatched <<)>>, <<]>>, or <<}>>,
or top-level <a class=production data-autolink=link><var><semicolon></var></a> tokens or <a class=production data-autolink=link><var><delim></var></a> tokens with a value of "!".
<p class=note> 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.
<p class=note> Note: Custom properties can contain a trailing <span class=css data-autolink=maybe>!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 data-autolink=maybe>!important</span> from being used,
as the <span class=css data-autolink=maybe>!important</span> is removed before syntax checking happens.
<p class=note> Note: While <a class=production data-autolink=link href=#any-value-production><var><any-value></var></a> must represent at least one token,
that one token may be whitespace.
This implies that <span class=css data-autolink=maybe>var-foo: ;</span> is valid,
and the corresponding <span class=css data-autolink=maybe>var(foo)</span> call would have a single space as its substitution value,
but <span class=css data-autolink=maybe>var-foo:;</span> is invalid.
<div class=example>
For example, the following is a valid custom property:
<pre>var-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.
</div>
<p> The values of custom properties,
and the values of <a data-autolink=link href=#cascading-variable>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.)
<p> The initial value of a <a data-autolink=link href=#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-autolink=maybe href=#variable-type>var()</a> notation,
which is explained in the section defining <a class=css data-autolink=maybe href=#variable-type>var()</a>.
<h3 data-level=2.2 id=using><span class=secno>2.2 </span>
Using Custom Properties</h3>
<p> The primary purpose of <a data-autolink=link href=#custom-property>custom properties</a> is to define <a data-autolink=link href=#cascading-variable>cascading variables</a>.
In CSS, a <dfn id=cascading-variable 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 data-autolink=link href=#custom-property>custom property</a> defines a corresponding <a data-autolink=link href=#cascading-variable>variable</a>
with the same name, minus the "var-" prefix.
For example, the custom property <a class=property data-autolink=property title=var-foo>var-foo</a> defines a variable named <span class=css data-autolink=maybe>foo</span>.
See the next chapter for details on how to use variables.
<p class=note> Note: <a data-autolink=link href=#custom-property>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>
<p> declares a <a data-autolink=link href=#custom-property>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>
<p> The preceding rule is equivalent to writing <span class=css data-autolink=maybe>background-color: #06c;</span>,
except that the variable name makes the origin of the color clearer,
and if <span class=css data-autolink=maybe>var(header-color)</span> is used on other elements in the document,
all of the uses can be updated at once
by changing the <a class=property data-autolink=property title=var-header-color>var-header-color</a> property on the root element.
</div>
<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 <span class=css data-autolink=maybe>@media</span> 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 data-autolink=link href=#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 { 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>
</pre> </div>
<h3 data-level=2.3 id=cycles><span class=secno>2.3 </span>
Resolving Dependency Cycles</h3>
<p> <a data-autolink=link href=#custom-property>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 data-autolink=link href=#custom-property>custom properties</a> each attempt to use the variable that the other defines;
doing so makes all the <a data-autolink=link href=#custom-property>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>
<p> The <a class=property data-autolink=property title=var-accent-background>var-accent-background</a> property
(along with any other properties that use <span class=css data-autolink=maybe>var(main-color)</span>)
will automatically update when the <a class=property data-autolink=property title=var-main-color>var-main-color</a> 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>
<p> Both <a class=property data-autolink=property title=var-one>var-one</a> and <a class=property data-autolink=property title=var-two>var-two</a> now define <a data-autolink=link href=#invalid-variable>invalid variables</a> rather than lengths.
</div>
<p> It is important to note that
<a data-autolink=link href=#custom-property>custom properties</a> resolve any <a data-autolink=link href=#cascading-variable>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><one><two><three /></two></one>
one { var-foo: 10px; }
two { var-bar: calc(var(foo) + 10px); }
three { var-foo: calc(var(bar) + 10px); }
</pre>
<p> The <one> element defines a value for <a class=property data-autolink=property title=var-foo>var-foo</a>.
The <two> element inherits this value,
and additionally assigns a value to <a class=property data-autolink=property title=var-bar>var-bar</a> using the <span class=css data-autolink=maybe>foo</span> variable.
Finally,
the <three> element inherits the <a class=property data-autolink=property title=var-bar>var-bar</a> value
<em>after</em> variable substitution
(in other words, it sees the value <span class=css data-autolink=maybe>calc(10px + 10px)</span>),
and then redefines <a class=property data-autolink=property title=var-foo>var-foo</a> in terms of that value.
Since the value it inherited for <a class=property data-autolink=property title=var-bar>var-bar</a> no longer contains a reference to the <a class=property data-autolink=property title=var-foo>var-foo</a> property defined on <one>,
defining <a class=property data-autolink=property title=var-foo>var-foo</a> using the <span class=css data-autolink=maybe>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 data-autolink=maybe>30px</span>.
</div>
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-->
<h2 data-level=3 id=using-variables><span class=secno>3 </span>
Using Cascading Variables: the <a class=css data-autolink=maybe href=#variable-type>var()</a> notation</h2>
<p> Every <a data-autolink=link href=#custom-property>custom property</a> automatically defines a corresponding <a data-autolink=link href=#cascading-variable>cascading variable</a>,
which can then be substituted into another property with the <a class=css data-autolink=maybe href=#variable-type>var()</a> function.
The syntax of <a class=css data-autolink=maybe href=#variable-type>var()</a> is:
<pre class=prod><dfn id=variable-type title="<variable>|var()"><variable></dfn> = var( <var>variable-name</var> [, <a class=production data-autolink=link href=#any-value-production><var><any-value></var></a> ]? )
</pre>
<p> 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.)
<p> 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.
<p class=note> Note: The syntax of the fallback, like that of <a data-autolink=link href=#custom-property>custom properties</a>, allows commas.
For example, <span class=css data-autolink=maybe>var(foo, red, blue)</span> defines a fallback of <span class=css data-autolink=maybe>red, blue</span>;
that is, anything between the first comma and the end of the function is considered a fallback value.
<p> 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 data-autolink=link href=#resolve-a-variable title="resolve a variable">resolved</a>.
<p> To <dfn id=resolve-a-variable>resolve a variable</dfn> in a property's value:
<ol>
<li>
If the variable named by the first argument to the <a class=css data-autolink=maybe href=#variable-type>var()</a> function is a valid variable,
replace the <a class=css data-autolink=maybe href=#variable-type>var()</a> function by the value of the corresponding <a data-autolink=link href=#custom-property>custom property</a>.
<li>
Otherwise, if the <a class=css data-autolink=maybe href=#variable-type>var()</a> function has a fallback value as its second argument,
replace the <a class=css data-autolink=maybe href=#variable-type>var()</a> function by the fallback value.
If there are any <a class=css data-autolink=maybe href=#variable-type>var()</a> references in the fallback,
<a data-autolink=link href=#resolve-a-variable title="resolve a variable">resolve</a> them as well.
<li>
Otherwise,
the property containing the <a class=css data-autolink=maybe href=#variable-type>var()</a> function is <a data-autolink=link href=#invalid-at-computed-value-time>invalid at computed-value time</a>.
<p class=note> Note: Other things can also make a property <a data-autolink=link href=#invalid-at-computed-value-time>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.
<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.
<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>
<p> This is <em>not</em> equivalent to setting <span class=css data-autolink=maybe>margin-top: 20px;</span>.
Instead, the second declaration is simply thrown away as a syntax error
for having an invalid property name.
<p> 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>
<p> Again, this is <em>not</em> equivalent to setting <span class=css data-autolink=maybe>margin-top: 20px;</span> (a length).
Instead, it's equivalent to <span class=css data-autolink=maybe>margin-top: 20 px;</span> (a number followed by an ident),
which is simply an invalid value for the <a class=property data-autolink=property title=margin-top>margin-top</a> property.
Note, though, that <span class=css data-autolink=maybe>calc()</span> can be used to validly achieve the same thing, like so:
<pre>.foo {
var-gap: 20;
margin-top: calc(var(gap) * 1px);
}
</pre> </div>
<p> 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 data-autolink=link href=#invalid-at-computed-value-time>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>
<p> Since <span class=css data-autolink=maybe>20px</span> is an invalid value for <a class=property data-autolink=property title=background-color>background-color</a>,
this instance of the property computes to <span class=css data-autolink=maybe>transparent</span>
(the initial value for <a class=property data-autolink=property title=background-color>background-color</a>)
instead.
<p> If the property was one that's inherited by default,
such as <a class=property data-autolink=property title=color>color</a>,
it would compute to the inherited value
rather than the initial value.
</div>
<h3 data-level=3.1 id=invalid-variables><span class=secno>3.1 </span>
Invalid Variables</h3>
<p> When a <a data-autolink=link href=#custom-property>custom property</a> has its initial value,
the <a data-autolink=link href=#cascading-variable>variable</a> it defines
represents an <dfn id=invalid-variable>invalid variable</dfn>.
Using an <a data-autolink=link href=#invalid-variable>invalid variable</a> in a property value
(including other <a data-autolink=link href=#custom-property>custom properties</a>)
makes the declaration <a data-autolink=link href=#invalid-at-computed-value-time>invalid at computed-value time</a>.
<p> A declaration can be <dfn id=invalid-at-computed-value-time>invalid at computed-value time</dfn>
if it uses an invalid <a data-autolink=link href=#cascading-variable>variable</a>, as explained above,
or if it uses a valid <a data-autolink=link href=#cascading-variable>variable</a>,
but the property value,
after substituting its <a data-autolink=link href=#cascading-variable>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>
<p> the <p> elements will have transparent backgrounds
(the initial value for <a class=property data-autolink=property title=background-color>background-color</a>),
rather than red backgrounds.
The same would happen if the variable itself was invalid.
<p> Note the difference between this
and what happens if the author had just written <span class=css data-autolink=maybe>background-color: 20px</span> directly in their stylesheet -
that would be a normal syntax error,
which would cause the rule to be discarded,
so the <span class=css data-autolink=maybe>background-color: red</span> rule would be used instead.
</div>
<p class=note> Note: The <a data-autolink=link href=#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.
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A:::::A A:::::A P::::::::P I::::::::I
AAAAAAA AAAAAAAPPPPPPPPPP IIIIIIIIII
-->
<h2 data-level=4 id=cssom><span class=secno>4 </span>
APIs</h2>
<h3 data-level=4.1 id=the-cssstyledeclaration-interface><span class=secno>4.1 </span>
Extensions to the <code>CSSStyleDeclaration</code> Interface</h3>
<p> The <dfn id=cssstyledeclaration><code>CSSStyleDeclaration</code></dfn> interface is amended as follows:
<pre class=idl>partial interface CSSStyleDeclaration {
readonly attribute CSSVariablesMap var;
}
</pre>
<p> A <code>CSSStyleDeclaration</code> is the <dfn id=associated-style-declaration>associated style declaration</dfn>
for the <code>CSSVariablesMap</code> assigned to its <code>var</code> attribute.
<p> 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 data-level=4.1.1 id=serializing-custom-props><span class=secno>4.1.1 </span>
Serializing Custom Properties</h4>
<p> 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 data-level=4.2 id=the-CSSVariablesMap-interface><span class=secno>4.2 </span>
The <code>CSSVariablesMap</code> Interface</h3>
<p> The <dfn id=cssvariablesmap><code>CSSVariablesMap</code></dfn> interface
exposes the <a data-autolink=link href=#custom-property>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>
<p> The <a href=http://dev.w3.org/2006/webapi/WebIDL/#MapClass>map entries</a>
on a CSSVariablesMap object
are the property names of all the <a data-autolink=link href=#custom-property>custom properties</a> in the <a href=http://dev.w3.org/csswg/cssom/#css-declaration-block-declarations><dfn id=css-declaration-block-declarations>CSS declaration block declarations</dfn></a>
with a non-initial value,
with the "var-" prefix removed,
paired with their values.
<h4 data-level=4.2.1 id=cssvariablesmap-methods><span class=secno>4.2.1 </span>
<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 data-autolink=link href=#associated-style-declaration>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>setProperty()</code>
on the <a data-autolink=link href=#associated-style-declaration>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 boolean
<dd>
Prepend "var-" to <var>varName</var>.
Invoke <code>removeProperty()</code>
on the <a data-autolink=link href=#associated-style-declaration>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>
<p> For example,
given the following style sheet:
<pre>div {
var-foo: 16px;
var-Bar: red;
var-foo-bar: 50%;
}
</pre>
<p> 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 <a class=property data-autolink=property title=var-foo>var-foo</a>.
<tr>
<td><code>el.style.var.get("Bar")</code>
<td><code>"red"</code>
<td>The value of <a class=property data-autolink=property title=var-Bar>var-Bar</a>.
<tr>
<td><code>el.style.var.get("foo-bar")</code>
<td><code>"50%"</code>
<td>The value of <a class=property data-autolink=property title=var-foo-bar>var-foo-bar</a>.
<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;
customProps.forEach(function(propValue, propName) {
if( knownCustomPropName(propName) ) {
/* Processing code here. */
}
});
</pre> </div>
<h2 data-level=5 id=changes><span class=secno>5 </span>
Changes from 10 April 2012 Working Draft</h2>
<p> <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 data-level=6 id=acknowledgments><span class=secno>6 </span>
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.
<h2 class="no-ref no-num" id=conformance>
Conformance</h2>
<h3 class=no-ref id=conventions>
Document conventions</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-autolink=biblio data-biblio-type=normative href=#rfc2119 title=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>
<h3 class=no-ref id=conformance-classes>
Conformance classes</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 id=partial>
Partial implementations</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 id=experimental>
Experimental implementations</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 id=testing>
Non-experimental implementations</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" id=references>
References</h2>
<h3 class="no-num no-ref" id=normative>
Normative References</h3>
<div data-fill-with=normative-references><dl><dt id=css21 title=CSS21>[CSS21]<dd>Bert Bos; et al. <a href=http://www.w3.org/TR/2011/REC-CSS2-20110607/>Cascading Style Sheets Level 2 Revision 1 (CSS 2.1) Specification</a>. 7 June 2011. W3C Recommendation. URL: <a href=http://www.w3.org/TR/2011/REC-CSS2-20110607/>http://www.w3.org/TR/2011/REC-CSS2-20110607/</a><dt id=rfc2119 title=RFC2119>[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></dl></div>
<h3 class="no-num no-ref" id=informative>
Informative References</h3>
<div data-fill-with=informative-references><dl><dt id=css3color title=CSS3COLOR>[CSS3COLOR]<dd>[; '; T; a; n; t; e; k; ; \; x; c; 3; \; x; 8; 7; e; l; i; k; '; ,; ; '; C; h; r; i; s; ; L; i; l; l; e; y; '; ,; ; '; L; .; ; D; a; v; i; d; ; B; a; r; o; n; '; ]. <a href=http://www.w3.org/TR/2011/REC-css3-color-20110607>CSS Color Module Level 3</a>. 7 June 2011. W3C Recommendation. URL: <a href=http://www.w3.org/TR/2011/REC-css3-color-20110607>http://www.w3.org/TR/2011/REC-css3-color-20110607</a></dl></div>
<h2 class="no-num no-ref" id=index>
Index</h2>
<div data-fill-with=index><ul class=indexlist><li><any-value>, <a href=#any-value-production title="section 2.1">2.1</a><li>associated style declaration, <a href=#associated-style-declaration title="section 4.1">4.1</a><li>cascading variable, <a href=#cascading-variable title="section 2.2">2.2</a><li>CSS declaration block declarations, <a href=#css-declaration-block-declarations title="section 4.2">4.2</a><li>CSSStyleDeclaration, <a href=#cssstyledeclaration title="section 4.1">4.1</a><li>CSSVariablesMap, <a href=#cssvariablesmap title="section 4.2">4.2</a><li>custom property, <a href=#custom-property 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>invalid variable, <a href=#invalid-variable title="section 3.1">3.1</a><li>resolve a variable, <a href=#resolve-a-variable title="section 3">3</a><li>var(), <a href=#variable-type title="section 3">3</a><li>var-*, <a href=#var- title="section 2">2</a><li><variable>, <a href=#variable-type title="section 3">3</a><li>variable, <a href=#cascading-variable title="section 2.2">2.2</a></ul></div>
<h2 class="no-num no-ref" id=property-index>
Property index</h2>
<div data-fill-with=property-index><table class=proptable><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>Computed value<th scope=col>Applies to<th scope=col>Animatable<tr><th scope=row><a data-property="">var-*</a><td><a class=production data-autolink=link><var><any-value></var></a><td>(nothing, see prose)<td><td>yes<td><td>all<td>specified value with variables substituted (but see prose for "invalid variables")<td>all elements<td>no</thead></table></div>