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<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01//EN" "http://www.w3.org/TR/html4/strict.dtd">
<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" type="text/css" rel="stylesheet">
<link href="../csslogo.ico" type="image/x-icon" rel="shortcut icon">
<link href="https://www.w3.org/StyleSheets/TR/W3C-ED.css" type="text/css" rel="stylesheet">
</head>
<body class="h-entry">
<div class="head">
<p data-fill-with="logo"><a href="http://www.w3.org/">
<img width="72" alt="W3C" src="http://www.w3.org/Icons/w3c_home" height="48">
</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="20130322">22 March 2013</span></span></h2>
<div data-fill-with="spec-metadata"><dl><dt>This version:</dt><dd><a href="http://dev.w3.org/csswg/css-variables/" class="u-url">http://dev.w3.org/csswg/css-variables/</a></dd><dt>Latest version:</dt><dd><a href="http://www.w3.org/TR/css-variables/">http://www.w3.org/TR/css-variables/</a></dd><dt>Editor's Draft:</dt><dd><a href="http://dev.w3.org/csswg/css-variables/">http://dev.w3.org/csswg/css-variables/</a></dd><dt>Previous Versions:</dt><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></dd><dt>Editors:
</dt><dd class="p-author h-card vcard"><a href="http://xanthir.com/contact" class="p-name fn u-url url">Tab Atkins Jr.</a> (<span class="p-org org">Google</span>)</dd><dd class="p-author h-card vcard"><span class="p-name fn">Luke Macpherson</span> (<span class="p-org org">Google</span>), <a href="mailto:macpherson@google.com" class="u-email email">macpherson@google.com</a></dd><dd class="p-author h-card vcard"><span class="p-name fn">Daniel Glazman</span> (<span class="p-org org">Disruptive Innovations</span>), <a href="mailto:daniel.glazman@disruptive-innovations.com" class="u-email email">daniel.glazman@disruptive-innovations.com</a></dd></dl></div>
<p data-fill-with="copyright" class="copyright"><span><a href="http://www.w3.org/Consortium/Legal/ipr-notice#Copyright" rel="license">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.</span></p>
<hr title="Separator for header">
</div>
<h2 id="abstract" class="no-num no-toc no-ref">Abstract</h2>
<p data-fill-with="abstract" class="p-summary"><span>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.</span></p>
<h2 id="status" class="no-num no-toc no-ref">Status of this document</h2>
<div data-fill-with="status"><div>
<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><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><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><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>.</p>
</div></div>
<h2 id="contents" class="no-num no-toc no-ref">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><li><a href="#values"><span class="secno">1.2 </span>
Values</a></li></ul></li><li><a href="#defining-variables"><span class="secno">2 </span>
Defining Custom Properties: the <span data-autolink="property" class="property" title="var-*">var-*</span> family of properties</a></li><li><a href="#using-variables"><span class="secno">3 </span>
Using Cascading Variables: the <span data-autolink="maybe" class="css">var()</span> notation</a><ul class="toc"><li><a href="#invalid-variables"><span class="secno">3.1 </span>
Invalid Variables</a></li></ul></li><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></li></ul></li><li><a href="#the-cssvariablesdeclaration-interface"><span class="secno">4.2 </span>
The <code>CSSVariablesDeclaration</code> Interface</a></li></ul></li><li><a href="#changes"><span class="secno">5 </span>
Changes from 10 April 2012 Working Draft</a></li><li><a href="#acknowledgments"><span class="secno">6 </span>
Acknowledgments</a></li><li><a href="#conformance">
Conformance</a><ul class="toc"><li><a href="#conventions">
Document conventions</a></li><li><a href="#conformance-classes">
Conformance classes</a></li><li><a href="#partial">
Partial implementations</a></li><li><a href="#experimental">
Experimental implementations</a></li><li><a href="#testing">
Non-experimental implementations</a></li></ul></li><li><a href="#references">
References</a><ul class="toc"><li><a href="#normative">
Normative References</a></li><li><a href="#informative">
Informative References</a></li></ul></li><li><a href="#index">
Index</a></li><li><a href="#property-index">
Property index</a></li></ul></div>
<h2 id="introduction" data-level="1"><span class="secno">1 </span>
Introduction</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-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.
</p><h3 id="module-interactions" data-level="1.1"><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.
</p><h3 id="values" data-level="1.2"><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" U00022="" data-biblio-type="normative" title="CSS21" href="#css21">[CSS21]</a>.
Value types not defined in this specification are defined in CSS Level 2 Revision 1 <a data-autolink="biblio" U00022="" data-biblio-type="normative" title="CSS21" href="#css21">[CSS21]</a>.
Other CSS modules may expand the definitions of these value types:
for example <a data-autolink="biblio" U00022="" data-biblio-type="informative" title="CSS3COLOR" href="#css3color">[CSS3COLOR]</a>,
when combined with this module,
expands the definition of the <color> value type as used in this specification.
</color></p><h2 id="defining-variables" data-level="2"><span class="secno">2 </span>
Defining Custom Properties: the <a data-autolink="property" class="property" title="var-*" href="#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>.
</p><table class="propdef"><tbody><tr><th>Name:</th><td><dfn id="var-">var-*</dfn></td></tr><tr><th>Values:</th><td>[ <a data-autolink="link" class="production" href="#value-production"><var><value></var></a> | <a data-autolink="link" class="production" href="#cdo-production"><var><CDO></var></a> | <a data-autolink="link" class="production" href="#cdc-production"><var><CDC></var></a> ]</td></tr><tr><th>Initial:</th><td>(nothing, see prose)</td></tr><tr><th>Applies To:</th><td>all elements</td></tr><tr><th>Inherited:</th><td>yes</td></tr><tr><th>Computed Value:</th><td>specified value with variables substituted (but see prose for "invalid variables")</td></tr><tr><th>Media:</th><td>all</td></tr></tbody></table>
<p> A <dfn id="custom-property">custom property</dfn> is any property
whose name is composed of "var-" followed by an <a data-autolink="link" class="production"><var><ident></var></a> <a data-autolink="biblio" U00022="" data-biblio-type="normative" title="CSS3VAL" href="#css3val">[CSS3VAL]</a>.
<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><div class="example">
Custom properties define variables,
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 tons of edits across all stylesheets in the project.
</p></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.
</p><div class="example">
For example, <a data-autolink="maybe" class="css">VAR-FOO</a> is invalid,
because the prefix isn't "var-".
<p> While both <a data-autolink="maybe" class="css">var-foo</a> and <a data-autolink="maybe" class="css">var-FOO</a> are valid,
they are distinct properties -
using <a data-autolink="maybe" class="css">var(foo)</a> will refer to the first one,
while using <a data-autolink="maybe" class="css">var(FOO)</a> will refer to the second.
</p></div>
<p> <a data-autolink="link" href="#custom-property">Custom properties</a> have an extremely permissive value grammar.
The <dfn id="value-production"><value></dfn> in its grammar corresponds to the "value" production
in <a href="http://www.w3.org/TR/CSS2/syndata.html#tokenization">CSS 2.1 Chapter 4.1</a> <a data-autolink="biblio" U00022="" data-biblio-type="normative" title="CSS21" href="#css21">[CSS21]</a>,
while <dfn id="cdo-production"><CDO></dfn> and <dfn id="cdc-production"><CDC></dfn> correspond to the tokens of the same name
from the same chapter
(they represent HTML comments showing up in CSS text - "<!--" and "-->").
This is a very technical way of saying that nearly anything can be used in the value of a custom property,
save unmatched closing brackets ("]", ")", or "}"),
a top-level semicolon (as it will end the property),
a "!important" that's not at the end,
or invalid tokens (such as BAD_STRING and BAD_URL).
</p><p class="note">
Custom properties can contain a trailing <a data-autolink="maybe" class="css">!important</a>,
but this is automatically removed from the property's value by the CSS parser,
and makes the custom property "important" in the CSS cascade.
</p><p> Further, the value of a custom property must retain its original author-given casing,
unlike most CSS values which can be safely lower-cased
(because most of CSS is case-insensitive in the ASCII range).
(This requirement does not apply when a custom property's value
is substituted into another property
via a variable.)
</p><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 title="">variable</em>,
as it would be invalid in any normal property,
it might be read and acted on by JavaScript.
</p></div>
<p> There are an infinity of <a data-autolink="link" href="#custom-property">custom properties</a>,
but the initial value of a <a data-autolink="link" href="#custom-property">custom property</a> is an empty invalid value.
This means that,
until a <a data-autolink="link" href="#custom-property">custom property</a> is explicitly defined otherwise by a style sheet,
it defines an <a data-autolink="link" href="#invalid-variable">invalid variable</a>.
</p><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 title="variable|cascading variable" id="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 data-autolink="property" class="property" title="var-foo">var-foo</a> defines a variable named <a data-autolink="maybe" class="css">foo</a>.
See the next chapter for details on how to use variables.
</p><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.
</p><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:
</p><pre>h1 { background-color: var(header-color); }
</pre>
<p> The preceding rule is equivalent to writing <a data-autolink="maybe" class="css">background-color: #06c;</a>,
except that the variable name makes the origin of the color clearer,
and if <a data-autolink="maybe" class="css">var(header-color)</a> is used on other elements in the document,
all of the uses can be updated at once
by changing the <a data-autolink="property" class="property" title="var-header-color">var-header-color</a> property on the root element.
</p></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 <a data-autolink="maybe" class="css">@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><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>
<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).
</p><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 data-autolink="property" class="property" title="var-accent-background">var-accent-background</a> property
(along with any other properties that use <a data-autolink="maybe" class="css">var(main-color)</a>)
will automatically update when the <a data-autolink="property" class="property" title="var-main-color">var-main-color</a> 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 {
var-one: calc(var(two) + 20px);
var-two: calc(var(one) - 20px);
}
</pre>
<p> Both <a data-autolink="property" class="property" title="var-one">var-one</a> and <a data-autolink="property" class="property" title="var-two">var-two</a> now define <a data-autolink="link" href="#invalid-variables">invalid variables</a> rather than lengths.
</p></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.
</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 { 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 data-autolink="property" class="property" title="var-foo">var-foo</a>.
The <two> element inherits this value,
and additionally assigns a value to <a data-autolink="property" class="property" title="var-bar">var-bar</a> using the <a data-autolink="maybe" class="css">foo</a> variable.
Finally,
the <three> element inherits the <a data-autolink="property" class="property" title="var-bar">var-bar</a> value
</three></two></one><em>after</em> variable substitution
(in other words, it sees the value <a data-autolink="maybe" class="css">calc(10px + 10px)</a>),
and then redefines <a data-autolink="property" class="property" title="var-foo">var-foo</a> in terms of that value.
Since the value it inherited for <a data-autolink="property" class="property" title="var-bar">var-bar</a> no longer contains a reference to the <a data-autolink="property" class="property" title="var-foo">var-foo</a> property defined on <one>,
defining <a data-autolink="property" class="property" title="var-foo">var-foo</a> using the <a data-autolink="maybe" class="css">var(bar)</a> variable is not cyclic,
and actually defines a value that will eventually
(when referenced as a variable in a normal property)
resolve to <a data-autolink="maybe" class="css">30px</a>.
</one></p></div>
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U::::::U U::::::U i::::i
U::::::U U::::::U iiii
UU:::::U U:::::UU
U:::::U U:::::U ssssssssss iiiiiiinnnn nnnnnnnn ggggggggg ggggg
U:::::D D:::::U ss::::::::::s i:::::in:::nn::::::::nn g:::::::::ggg::::g
U:::::D D:::::Uss:::::::::::::s i::::in::::::::::::::nn g:::::::::::::::::g
U:::::D D:::::Us::::::ssss:::::s i::::inn:::::::::::::::ng::::::ggggg::::::gg
U:::::D D:::::U s:::::s ssssss i::::i n:::::nnnn:::::ng:::::g g:::::g
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U:::::D D:::::U s::::::s i::::i n::::n n::::ng:::::g g:::::g
U::::::U U::::::Ussssss s:::::s i::::i n::::n n::::ng::::::g g:::::g
U:::::::UUU:::::::Us:::::ssss::::::si::::::i n::::n n::::ng:::::::ggggg:::::g
UU:::::::::::::UU s::::::::::::::s i::::::i n::::n n::::n g::::::::::::::::g
UU:::::::::UU s:::::::::::ss i::::::i n::::n n::::n gg::::::::::::::g
UUUUUUUUU sssssssssss iiiiiiii nnnnnn nnnnnn gggggggg::::::g
g:::::g
gggggg g:::::g
g:::::gg gg:::::g
g::::::ggg:::::::g
gg:::::::::::::g
ggg::::::ggg
gggggg
-->
<h2 id="using-variables" data-level="3"><span class="secno">3 </span>
Using Cascading Variables: the <a data-autolink="maybe" class="css">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 data-autolink="maybe" class="css">var()</a> function.
The syntax of <a data-autolink="maybe" class="css">var()</a> is:
</p><pre><dfn id="default-var-type"><variable></dfn> = var( <var>variable-name</var> [, <var><fallback></var> ]? )
</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><p> The <var><fallback></var> value is identical to the syntax of a <a data-autolink="link" href="#custom-property">custom property</a> value.
</p><p> If the variable named by the first argument is valid,
the variable's value is substituted as normal.
If the variable is invalid,
and a <var><fallback></var> was provided,
the <var><fallback></var> is substituted instead.
Otherwise, the result of evaluating the <a data-autolink="maybe" class="css">var()</a> function will mean
that the containing declaration is invalid at computed-value time.
</p><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 {
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 <a data-autolink="maybe" class="css">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 {
var-gap: 20;
margin-top: var(gap)px;
}
</pre>
<p> Again, this is <em>not</em> equivalent to setting <a data-autolink="maybe" class="css">margin-top: 20px;</a> (a length).
Instead, it's equivalent to <a data-autolink="maybe" class="css">margin-top: 20 px;</a> (a number followed by an ident),
which is simply an invalid value for the <a data-autolink="property" class="property" title="margin-top">margin-top</a> property.
Note, though, that <a data-autolink="maybe" class="css">calc()</a> can be used to validly achieve the same thing, like so:
</p><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>.
</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 { var-looks-valid: 20px; }
p { background-color: var(looks-valid); }
</pre>
<p> Since <a data-autolink="maybe" class="css">20px</a> is an invalid value for <a data-autolink="property" class="property" title="background-color">background-color</a>,
this instance of the property computes to <a data-autolink="property" class="property" title="transparent">transparent</a>
(the initial value for <a data-autolink="property" class="property" title="background-color">background-color</a>)
instead.
</p><p> If the property was one that's inherited by default,
such as <a data-autolink="property" class="property" title="color">color</a>,
it would compute to the inherited value
rather than the initial value.
</p></div>
<p> In some cases,
it can be useful to provide a "default" value for a variable
in case the variable isn't defined or is invalid.
</p><p> For example, if a site uses variables to provide "hooks" for customization,
expecting the variables to be defined in a separate custom stylesheet,
the main stylesheet can use default values for its variable
so that the theming stylesheet can just override the variables it cares about,
rather than being forced to provide values for all of them.
</p><h3 id="invalid-variables" data-level="3.1"><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><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.
</p><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 data-autolink="property" class="property" title="background-color">background-color</a>),
rather than red backgrounds.
The same would happen if the variable itself was invalid.
</p></p><p> Note the difference between this
and what happens if the author had just written <a data-autolink="maybe" class="css">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 data-autolink="maybe" class="css">background-color: red</a> rule would be used instead.
</p></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.
<!--
AAA PPPPPPPPPPPPPPPPP IIIIIIIIII
A:::A P::::::::::::::::P I::::::::I
A:::::A P::::::PPPPPP:::::P I::::::::I
A:::::::A PP:::::P P:::::PII::::::II
A:::::::::A P::::P P:::::P I::::I
A:::::A:::::A P::::P P:::::P I::::I
A:::::A A:::::A P::::PPPPPP:::::P I::::I
A:::::A A:::::A P:::::::::::::PP I::::I
A:::::A A:::::A P::::PPPPPPPPP I::::I
A:::::AAAAAAAAA:::::A P::::P I::::I
A:::::::::::::::::::::A P::::P I::::I
A:::::AAAAAAAAAAAAA:::::A P::::P I::::I
A:::::A A:::::A PP::::::PP II::::::II
A:::::A A:::::A P::::::::P I::::::::I
A:::::A A:::::A P::::::::P I::::::::I
AAAAAAA AAAAAAAPPPPPPPPPP IIIIIIIIII
-->
</p><h2 id="cssom" data-level="4"><span class="secno">4 </span>
APIs</h2>
<h3 id="the-cssstyledeclaration-interface" data-level="4.1"><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:
</p><pre class="idl">partial interface CSSStyleDeclaration {
attribute CSSVariablesDeclaration var;
}
</pre>
<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.
</p><h4 id="serializing-custom-props" data-level="4.1.1"><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><p class="note">
Ordinarily, property names are restricted to the ASCII range and are case-insensitive,
so implementations typically serialize the name lowercased.
</p><h3 id="the-cssvariablesdeclaration-interface" data-level="4.2"><span class="secno">4.2 </span>
The <code>CSSVariablesDeclaration</code> Interface</h3>
<p> The <dfn id="cssvariablesdeclaration"><code>CSSVariablesDeclaration</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.
</p><pre class="idl">interface CSSVariablesDeclaration {
<a data-autolink="link" href="#cssvariablesdeclaration-getter">getter</a> DOMString (DOMString varName);
<a data-autolink="link" href="#cssvariablesdeclaration-setter">setter</a> <a data-autolink="link" href="#cssvariablesdeclaration-creator">creator</a> void (DOMString varName, DOMString varValue);
<a data-autolink="link" href="#cssvariablesdeclaration-deleter">deleter</a> void (DOMString varName);
}
</pre>
<p> The <a href="http://dev.w3.org/2006/webapi/WebIDL/#dfn-supported-property-names">supported property names</a>
on a CSSStyleDeclaration 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.
</p><p> Before running any of the algorithms in this section,
prepend "var-" to <var>varName's</var> value.
</p><p> When asked to <dfn id="cssvariablesdeclaration-getter" title="getter">get the value of a variable</dfn>,
if <var>varName</var> is in the <a data-autolink="link" href="#css-declaration-block-declarations">CSS declaration block declarations</a>,
invoke <code>getPropertyValue()</code> by passing <var>varName</var> as its argument,
and return the returned value.
Otherwise, return the empty string.
</p><p> When asked to <dfn id="cssvariablesdeclaration-setter" title="setter">set</dfn> or <dfn id="cssvariablesdeclaration-creator" title="creator">create the value of a variable</dfn>,
invoke <code>setProperty()</code> by passing <var>varName</var> as the <var>property</var> argument and <var>varValue</var> as the <var>value</var> argument.
</p><p class="note">
Note that using <code>setProperty()</code> to set a property to the empty string
instead deletes the property.
</p><p> When asked to <dfn id="cssvariablesdeclaration-deleter" title="deleter">delete the value of a variable</dfn>,
if <var>varName</var> matches the grammar of a <a data-autolink="link" href="#custom-property">custom property</a> name,
invoke <code>removeProperty()</code> by passing <var>varName</var> as its argument,
and return the returned value.
Otherwise, do nothing and return the empty string.
</p><div class="example">
For example,
given the following style sheet:
<pre>div {
var-foo: 16px;
var-Bar: red;
var-foo-bar: 50%;
}
</pre>
<p> The following lines of script all return something useful:
</p><pre>el = document.querySelector("div");
print(el.style.var.foo); /* Prints the value of "var-foo" */
print(el.style.var.Bar); /* Prints the value of "var-Bar" */
print(el.style.var["foo-bar"]); /* Prints the value of "var-foo-bar" */
</pre>
<p> On the other hand, the following do not:
</p><pre>print(el.style.varFoo);
/* Custom properties don't exist directly on "style" */
print(el.style.varfoo);
/* Not even if the casing matches. */
print(el.style.var.foo-bar);
/* Retrieves "var-foo" and subtracts a JS variable named "bar",
rather than retrieving the value of "var-foo-bar" */
</pre> </div>
<div class="example">
Iterating over all of the custom properties
(for example, for a JS library to find the ones it knows about and wants to respond to)
is also simple with the <code>var</code> property:
<pre>var customProps = el.style.var;
for(customPropName in customProps) {
var customPropValue = customProps[customPropName];
/* More stuff here. */
}
</pre> </div>
<h2 id="changes" data-level="5"><span class="secno">5 </span>
Changes from 10 April 2012 Working Draft</h2>
<p> </p><ul>
<li>The value syntax for custom properties has been nailed down more precisely.
</li><li>Case-sensitivity of custom property names has been defined.
</li><li>The fallback argument was added to the var() function.
</li><li>A property that is invalid at computed-value time now either goes inherit or initial, rather than always initial.
</li><li>CSSVariableComponentValue interface has been dropped, pending the *ComponentValue interfaces being created at all.
</li><li>The CSSVariablesDeclaration interface has been added, which stores all the variables defined by a style rule.
</li></ul>
<h2 id="acknowledgments" data-level="6"><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.
</p><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><p>All of the text of this specification is normative except sections
explicitly marked as non-normative, examples, and notes. <a data-autolink="biblio" U00022="" data-biblio-type="normative" title="RFC2119" href="#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:
</p><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><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:
</p><dl>
<dt><dfn title="style sheet!!as conformance class" id="style-sheet">style sheet</dfn>
</dt><dd>A <a href="http://www.w3.org/TR/CSS21/conform.html#style-sheet">CSS
style sheet</a>.
</dd><dt><dfn id="renderer">renderer</dfn></dt>
<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.
</dd><dt><dfn id="authoring-tool">authoring tool</dfn></dt>
<dd>A <a href="http://www.w3.org/TR/CSS21/conform.html#user-agent">UA</a>
that writes a style sheet.
</dd></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><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><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.
</p><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><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><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><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.
</p><h2 id="references" class="no-num no-ref">
References</h2>
<h3 id="normative" class="no-num no-ref">
Normative References</h3>
<div data-fill-with="normative-references"><dl><dt id="css3val" title="CSS3VAL">[CSS3VAL]</dt><dd>Håkon Wium Lie; Tab Atkins; Elika J. Etemad. <a href="http://www.w3.org/TR/2012/CR-css3-values-20120828/">CSS Values and Units Module Level 3</a>. 28 August 2012. W3C Candidate Recommendation. (Work in progress.) URL: <a href="http://www.w3.org/TR/2012/CR-css3-values-20120828/">http://www.w3.org/TR/2012/CR-css3-values-20120828/</a></dd><dt id="css21" title="CSS21">[CSS21]</dt><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></dd><dt id="rfc2119" title="RFC2119">[RFC2119]</dt><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 id="informative" class="no-num no-ref">
Informative References</h3>
<div data-fill-with="informative-references"><dl><dt id="css3color" title="CSS3COLOR">[CSS3COLOR]</dt><dd>Tantek Çelik; Chris Lilley; L. David Baron. <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></dd></dl></div>
<h2 id="index" class="no-num no-ref">
Index</h2>
<div data-fill-with="index"><ul class="indexlist"><li>authoring tool, <a href="#authoring-tool" title="section ??">??</a></li><li>cascading variable, <a href="#cascading-variable" title="section 2">2</a></li><li><CDC>, <a href="#cdc-production" title="section 2">2</a></li><li><CDO>, <a href="#cdo-production" title="section 2">2</a></li><li>creator, <a href="#cssvariablesdeclaration-creator" title="section 4.2">4.2</a></li><li>CSS declaration block declarations, <a href="#css-declaration-block-declarations" title="section 4.2">4.2</a></li><li>CSSStyleDeclaration, <a href="#cssstyledeclaration" title="section 4.1">4.1</a></li><li>CSSVariablesDeclaration, <a href="#cssvariablesdeclaration" title="section 4.2">4.2</a></li><li>custom property, <a href="#custom-property" title="section 2">2</a></li><li>deleter, <a href="#cssvariablesdeclaration-deleter" title="section 4.2">4.2</a></li><li>getter, <a href="#cssvariablesdeclaration-getter" title="section 4.2">4.2</a></li><li>invalid at computed-value time, <a href="#invalid-at-computed-value-time" title="section 3.1">3.1</a></li><li>invalid variable, <a href="#invalid-variable" title="section 3.1">3.1</a></li><li>renderer, <a href="#renderer" title="section ??">??</a></li><li>setter, <a href="#cssvariablesdeclaration-setter" title="section 4.2">4.2</a></li><li>style sheet!!as conformance class, <a href="#style-sheet" title="section ??">??</a></li><li><value>, <a href="#value-production" title="section 2">2</a></li><li>var-*, <a href="#var-" title="section 2">2</a></li><li><variable>, <a href="#default-var-type" title="section 3">3</a></li><li>variable, <a href="#cascading-variable" title="section 2">2</a></li></ul></div>
<h2 id="property-index" class="no-num no-ref">
Property index</h2>
<div data-fill-with="property-index"><table class="proptable"><thead><tr><th scope="col">Name</th><th scope="col">Values</th><th scope="col">Initial</th><th scope="col">Applies To</th><th scope="col">Inh.</th><th scope="col">%ages</th><th scope="col">Media</th><th scope="col">Computed Value</th></tr><tr><th scope="row"><a data-property="" data-autolink="link" href="#var-">var-*</a></th><td>[ <a data-autolink="link" class="production" href="#value-production"><var><value></var></a> | <a data-autolink="link" class="production" href="#cdo-production"><var><CDO></var></a> | <a data-autolink="link" class="production" href="#cdc-production"><var><CDC></var></a> ]</td><td>(nothing, see prose)</td><td>all elements</td><td>yes</td><td></td><td>all</td><td>specified value with variables substituted (but see prose for "invalid variables")</td></tr></thead></table></div>
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