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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 profile="http://dublincore.org/documents/2008/08/04/dc-html/ ">
<meta content="text/html; charset=utf-8" http-equiv=Content-Type>
<title>CSS Custom Properties for Cascading Variables Module Level 1</title>
<link href="http://purl.org/dc/terms/" rel=schema.dcterms>
<link href="http://www.w3.org/Consortium/Legal/ipr-notice#Copyright"
rel=dcterms.rights>
<meta
content="CSS Custom Properties for Cascading Variables Module Level 1"
name=dcterms.title>
<meta content=text name=dcterms.type>
<meta content=2013-03-06 name=dcterms.issued>
<meta content="http://dev.w3.org/csswg/css-variables/"
name=dcterms.creator>
<meta content=W3C name=dcterms.publisher>
<meta content="http://dev.w3.org/csswg/css-variables/"
name=dcterms.identifier>
<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.css" rel=stylesheet
type="text/css">
<body>
<div class=head> <!--begin-logo-->
<p><a href="http://www.w3.org/"><img alt=W3C height=48
src="http://www.w3.org/Icons/w3c_home" width=72></a> <!--end-logo-->
<h1 class=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 6 March 2013</h2>
<dl>
<dt>This version:
<dd><a
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/">http://www.w3.org/TR/css-variables/</a>
<dt>Editor's Draft
<dd><a
href="http://dev.w3.org/csswg/css-variables/">http://dev.w3.org/csswg/css-variables/</a>
<dt>Editors:
<dd class=hcard><a class="fn url" href="http://xanthir.com/contact">Tab
Atkins Jr.</a> (<span class=org>Google</span>)
<dd class=hcard><span class=fn>Luke Macpherson</span> (<span
class=org>Google</span>), <a class=email
href="macpherson@google.com">macpherson@google.com </a>
<dd class=hcard><a class=email href="macpherson@google.com"><span
class=fn>Daniel Glazman</span> (<span class=org>Disruptive
Innovations</span>), </a><a class=email
href="daniel.glazman@disruptive-innovations.com">daniel.glazman@disruptive-innovations.com
</a>
</dl>
<!--begin-copyright-->
<p class=copyright><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.</p>
<!--end-copyright--><a class=email
href="daniel.glazman@disruptive-innovations.com"> </a>
<hr title="Separator for header">
</div>
<p><a class=email href="daniel.glazman@disruptive-innovations.com"> </a>
<h2 class="no-num no-toc no-ref" id=abstract>Abstract</h2>
<p> This module introduces cascading variables as a new primitive value
type that is accepted by all CSS properties, and custom properties for
defining them.
<h2 class="no-num no-toc no-ref" id=status>Status of this document</h2>
<!--begin-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>.</p>
<!--end-status-->
<h2 class="no-num no-toc no-ref" id=contents> Table of contents</h2>
<!--begin-toc-->
<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 ‘<code class=css>var-*</code>’ family of
properties</a>
<li><a href="#using-variables"><span class=secno>3. </span> Using
Cascading Variables: the ‘<code class=css>var()</code>’ 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-cssvariablesdeclaration-interface"><span
class=secno>4.2. </span> The <code>CSSVariablesDeclaration</code>
Interface</a>
</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 class=no-ref><a href="#conformance"><span class=secno>7. </span>
Conformance</a>
<ul class=toc>
<li class=no-ref><a href="#conventions"><span class=secno>7.1. </span>
Document conventions</a>
<li class=no-ref><a href="#conformance-classes"><span class=secno>7.2.
</span> Conformance classes</a>
<li class=no-ref><a href="#partial"><span class=secno>7.3. </span>
Partial implementations</a>
<li class=no-ref><a href="#experimental"><span class=secno>7.4. </span>
Experimental implementations</a>
<li class=no-ref><a href="#testing"><span class=secno>7.5. </span>
Non-experimental implementations</a>
</ul>
<li class="no-num no-ref"><a href="#references"> References</a>
<ul class=toc>
<li class="no-num no-ref"><a href="#normative-references"> Normative
references</a>
<li class="no-num no-ref"><a href="#other-references"> Other
references</a>
</ul>
<li class="no-num no-ref"><a href="#index"> Index</a>
<li class="no-num no-ref"><a href="#property-index"> Property index</a>
</ul>
<!--end-toc-->
<h2 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><a class=email href="daniel.glazman@disruptive-innovations.com"> This
module introduces a family of custom author-defined properties known
collectively as </a><a href="#custom-property">custom properties</a>,
which allow an author to assign arbitrary values to a property with an
author-chosen name, and <a 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 href="#custom-property">custom property</a>, and the
change will propagate to all uses of that variable automatically.
<h3 id=module-interactions><span class=secno>1.1. </span> Module
Interactions</h3>
<p> This module defines a new type of primitive value, the <a
href="#cascading-variable">variable</a>, which is accepted by all
properties.
<h3 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 href="#CSS21"
rel=biblioentry>[CSS21]<!--{{!CSS21}}--></a>. Value types not defined in
this specification are defined in CSS Level 2 Revision 1 <a
href="#CSS21" rel=biblioentry>[CSS21]<!--{{!CSS21}}--></a>. Other CSS
modules may expand the definitions of these value types: for example <a
href="#CSS3COLOR" rel=biblioentry>[CSS3COLOR]<!--{{CSS3COLOR}}--></a>,
when combined with this module, expands the definition of the
<color> value type as used in this specification.
<h2 id=defining-variables><span class=secno>2. </span> Defining Custom
Properties: the ‘<a href="#var-"><code class=css>var-*</code></a>’
family of properties</h2>
<p> This specification defines an open-ended set of properties called <a
href="#custom-property">custom properties</a>, which are used to define <a
href="#cascading-variable">variables</a>.
<table class=propdef>
<tbody>
<tr>
<th>Name:
<td><dfn id=var->var-*</dfn>
<tr>
<th>Values:
<td>[ <a href="#value-production"><value></a> | <a
href="#cdo-production"><CDO></a> | <a
href="#cdc-production"><CDC></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
</table>
<p> A <dfn id=custom-property
title="custom property|custom properties">custom property</dfn> is any
property whose name is composed of "var-" followed by an
<code><ident></code> <a href="#CSS3VAL"
rel=biblioentry>[CSS3VAL]<!--{{!CSS3VAL}}--></a> <a
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.
<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.
</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, ‘<code class=css>VAR-FOO</code>’ is
invalid, because the prefix isn't "var-".
<p> While both ‘<code class=css>var-foo</code>’ and ‘<code
class=css>var-FOO</code>’ are valid, they are distinct properties -
using ‘<code class=css>var(foo)</code>’ will refer to the first one,
while using ‘<code class=css>var(FOO)</code>’ will refer to the
second.
</div>
<p> <a 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 href="#CSS21" rel=biblioentry>[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 class=note> Custom properties can contain a trailing ‘<code
class=css>!important</code>’, 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> 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.)
<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.
</div>
<p> There are an infinity of <a href="#custom-property">custom
properties</a>, but the initial value of a <a
href="#custom-property">custom property</a> is an empty invalid value.
This means that, until a <a href="#custom-property">custom property</a> is
explicitly defined otherwise by a style sheet, it defines an <a
href="#invalid-variable">invalid variable</a>.
<p> The primary purpose of <a href="#custom-property">custom properties</a>
is to define <a 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 href="#custom-property">custom property</a> defines a
corresponding <a href="#cascading-variable">variable</a> with the same
name, minus the "var-" prefix. For example, the custom property ‘<code
class=property>var-foo</code>’ defines a variable named ‘<code
class=css>foo</code>’. See the next chapter for details on how to use
variables.
<p class=note> Note: <a 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 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 ‘<code
class=css>background-color: #06c;</code>’, except that the variable
name makes the origin of the color clearer, and if ‘<code
class=css>var(header-color)</code>’ is used on other elements in the
document, all of the uses can be updated at once by changing the ‘<code
class=property>var-header-color</code>’ 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 ‘<code class=css>@media</code>’ 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 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 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 href="#custom-property">custom
properties</a> each attempt to use the variable that the other defines;
doing so makes all the <a 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 ‘<code class=property>var-accent-background</code>’ property
(along with any other properties that use ‘<code
class=css>var(main-color)</code>’) will automatically update when the
‘<code class=property>var-main-color</code>’ 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 ‘<code class=property>var-one</code>’ and ‘<code
class=property>var-two</code>’ now define <a
href="#invalid-variables">invalid variables</a> rather than lengths.
</div>
<p> It is important to note that <a href="#custom-property">custom
properties</a> resolve any <a 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 ‘<code
class=property>var-foo</code>’. The <two> element inherits this
value, and additionally assigns a value to ‘<code
class=property>var-bar</code>’ using the ‘<code
class=css>foo</code>’ variable. Finally, the <three> element
inherits the ‘<code class=property>var-bar</code>’ value
<em>after</em> variable substitution (in other words, it sees the value
‘<code class=css>calc(10px + 10px)</code>’), and then redefines
‘<code class=property>var-foo</code>’ in terms of that value. Since
the value it inherited for ‘<code class=property>var-bar</code>’ no
longer contains a reference to the ‘<code
class=property>var-foo</code>’ property defined on <one>,
defining ‘<code class=property>var-foo</code>’ using the ‘<code
class=css>var(bar)</code>’ variable is not cyclic, and actually defines
a value that will eventually (when referenced as a variable in a normal
property) resolve to ‘<code class=css>30px</code>’.
</div>
<!--
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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
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U:::::D D:::::U s::::::s i::::i n::::n n::::ng:::::g g:::::g
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
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-->
<h2 id=using-variables><span class=secno>3. </span> Using Cascading
Variables: the ‘<code class=css>var()</code>’ notation</h2>
<p> Every <a href="#custom-property">custom property</a> automatically
defines a corresponding <a href="#cascading-variable">cascading
variable</a>, which can then be substituted into another property with the
‘<code class=css>var()</code>’ function. The syntax of ‘<code
class=css>var()</code>’ is:
<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> The <var><fallback></var> value is identical to the syntax of a
<a href="#custom-property">custom property</a> value.
<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 ‘<code
class=css>var()</code>’ function will mean that the containing
declaration is invalid at computed-value time.
<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 ‘<code
class=css>margin-top: 20px;</code>’. 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 ‘<code
class=css>margin-top: 20px;</code>’ (a length). Instead, it's
equivalent to ‘<code class=css>margin-top: 20 px;</code>’ (a number
followed by an ident), which is simply an invalid value for the ‘<code
class=property>margin-top</code>’ property. Note, though, that ‘<code
class=css>calc()</code>’ 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
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 ‘<code class=css>20px</code>’ is an invalid value for
‘<code class=property>background-color</code>’, this instance of the
property computes to ‘<code class=property>transparent</code>’ (the
initial value for ‘<code class=property>background-color</code>’)
instead.
<p> If the property was one that's inherited by default, such as ‘<code
class=property>color</code>’, it would compute to the inherited value
rather than the initial value.
</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> 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.
<h3 id=invalid-variables><span class=secno>3.1. </span> Invalid Variables</h3>
<p> When a <a href="#custom-property">custom property</a> has its initial
value, the <a href="#cascading-variable">variable</a> it defines
represents an <dfn id=invalid-variable>invalid variable</dfn>. Using an <a
href="#invalid-variable">invalid variable</a> in a property value
(including other <a href="#custom-property">custom properties</a>) makes
the declaration <a 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
href="#cascading-variable">variable</a>, as explained above, or if it uses
a valid <a href="#cascading-variable">variable</a>, but the property
value, after substituting its <a 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 ‘<code class=property>background-color</code>’), 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 ‘<code class=css>background-color: 20px</code>’ directly
in their stylesheet - that would be a normal syntax error, which would
cause the rule to be discarded, so the ‘<code
class=css>background-color: red</code>’ rule would be used instead.
</div>
<p class=note> Note: The <a 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
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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
-->
<h2 id=cssom><span class=secno>4. </span> APIs</h2>
<h3 id=the-cssstyledeclaration-interface><span class=secno>4.1. </span>
Extensions to the <a
href="#cssstyledeclaration"><code>CSSStyleDeclaration</code></a> Interface</h3>
<p> The <dfn id=cssstyledeclaration><code>CSSStyleDeclaration</code></dfn>
interface is amended as follows:
<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.
<h4 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 id=the-cssvariablesdeclaration-interface><span class=secno>4.2. </span>
The <a
href="#cssvariablesdeclaration"><code>CSSVariablesDeclaration</code></a>
Interface</h3>
<p> The <dfn
id=cssvariablesdeclaration><code>CSSVariablesDeclaration</code></dfn>
interface exposes the <a href="#custom-property">custom properties</a>
declared in the parent declaration block that have a non-initial value.
<pre class=idl>interface CSSVariablesDeclaration {
<a
href="#cssvariablesdeclaration-getter">getter</a> DOMString (DOMString varName);
<a
href="#cssvariablesdeclaration-setter">setter</a> <a
href="#cssvariablesdeclaration-creator">creator</a> void (DOMString varName, DOMString varValue);
<a
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 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> Before running any of the algorithms in this section, prepend "var-" to
<var>varName's</var> value.
<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
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> 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 class=note> Note that using <code>setProperty()</code> to set a property
to the empty string instead deletes the property.
<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 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.
<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:
<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:
<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><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 CSSVariablesDeclaration interface has been added, which stores all
the variables defined by a style rule.
</ul>
<h2 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 id=conformance><span class=secno>7. </span> Conformance</h2>
<h3 class=no-ref id=conventions><span class=secno>7.1. </span> 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
href="#RFC2119" rel=biblioentry>[RFC2119]<!--{{!RFC2119}}--></a>
<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.
</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.
<h3 class=no-ref id=conformance-classes><span class=secno>7.2. </span>
Conformance classes</h3>
<p>Conformance to this specification is defined for three conformance
classes:
<dl>
<dt><dfn id=style-sheet title="style sheet!!as conformance class">style
sheet</dfn>
<dd>A <a href="http://www.w3.org/TR/CSS21/conform.html#style-sheet">CSS
style sheet</a>.
<dt><dfn id=renderer>renderer</dfn>
<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><dfn id=authoring-tool>authoring tool</dfn>
<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><span class=secno>7.3. </span> 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.
<h3 class=no-ref id=experimental><span class=secno>7.4. </span>
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.
<h3 class=no-ref id=testing><span class=secno>7.5. </span> Non-experimental
implementations</h3>