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<!DOCTYPE html>
<html lang=en>
<head><meta content="text/html; charset=utf-8" http-equiv=Content-Type>
<title>CSS Variables Module Level 1</title>
<link href="http://dev.w3.org/csswg/default.css" rel=stylesheet
type="text/css">
<link href="http://www.w3.org/StyleSheets/TR/W3C-WD.css" rel=stylesheet
type="text/css">
<style>
pre.idl { border:solid thin; background:#eee; color:#000; padding:0.5em }
pre.idl :link, pre.idl :visited { color:inherit; background:transparent }
</style>
<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>CSS Variables Module Level 1</h1>
<!--<h2 class="no-num no-toc">W3C Working Draft 22 March 2012</h2>-->
<h2 class="no-num no-toc" id=w3c-working-draft-15-march-2012>W3C Working
Draft 15 March 2012</h2>
<dl>
<dt>This version:
<dd><a
href="http://www.w3.org/TR/2012/WD-css-variables-20120315/">http://www.w3.org/TR/2012/WD-css-variables-20120315/</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=vcard><span class=fn>Luke Macpherson</span>, <span
class=org>Google, Inc.</span>, <span
class=email>macpherson@google.com</span>
<dd class=vcard> <a class=url href="http://xanthir.com/contact/"> <span
class=fn>Tab Atkins Jr.</span>, </a> <span class=org>Google, Inc.</span>
<dd class=vcard> <span class=fn>Daniel Glazman</span>, <span
class=org>Disruptive Innovations</span>, <span
class=email>daniel.glazman@disruptive-innovations.com</span>
</dl>
<!--begin-copyright-->
<p class=copyright><a
href="http://www.w3.org/Consortium/Legal/ipr-notice#Copyright"
rel=license>Copyright</a> © 2012 <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>), 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-->
<hr title="Separator for header">
</div>
<h2 class="no-num no-toc" id=abstract>Abstract</h2>
<p>CSS is a language for describing the rendering of structured documents
(such as HTML and XML) on screen, on paper, in speech, etc. This module
contains the features of CSS level 3 relating to variables. It
includes and extends the functionality of CSS level 2 <a
href="#CSS21" rel=biblioentry>[CSS21]<!--{{!CSS21}}--></a>, which builds
on CSS level 1 <a href="#CSS1"
rel=biblioentry>[CSS1]<!--{{CSS1}}--></a>. The main extensions compared to
level 2 are the introduction of the variable as a new primitive value
type that is accepted by all properties.
<h2 class="no-num no-toc" id=status>Status of this document</h2>
<!--begin-status-->
<p><em>This section describes the status of this document at the time of
its publication. Other documents may supersede this document. A list of
current W3C publications and the latest revision of this technical report
can be found in the <a href="http://www.w3.org/TR/">W3C technical reports
index at http://www.w3.org/TR/.</a></em>
<p>Publication as a Working Draft does not imply endorsement by the W3C
Membership. This is a draft document and may be updated, replaced or
obsoleted by other documents at any time. It is inappropriate to cite this
document as other than 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">www-style@w3.org</a> (see
<a href="http://www.w3.org/Mail/Request">instructions</a>) is preferred
for discussion of this specification. When sending e-mail, please put the
text “css-variables” in the subject, preferably like this:
“[<!---->css-variables<!---->] <em>…summary of
comment…</em>”
<p>This document was produced by the <a
href="http://www.w3.org/Style/CSS/members">CSS Working Group</a> (part of
the <a href="http://www.w3.org/Style/">Style Activity</a>).
<p>This document was produced by a group operating under the <a
href="http://www.w3.org/Consortium/Patent-Policy-20040205/">5 February
2004 W3C Patent Policy</a>. W3C maintains a <a
href="http://www.w3.org/2004/01/pp-impl/32061/status"
rel=disclosure>public list of any patent disclosures</a> made in
connection with the deliverables of the group; that page also includes
instructions for disclosing a patent. An individual who has actual
knowledge of a patent which the individual believes contains <a
href="http://www.w3.org/Consortium/Patent-Policy-20040205/#def-essential">Essential
Claim(s)</a> must disclose the information in accordance with <a
href="http://www.w3.org/Consortium/Patent-Policy-20040205/#sec-Disclosure">section
6 of the W3C Patent Policy</a>.</p>
<!--end-status-->
<!--<p>The following features are at risk: …</p>-->
<h2 class="no-num no-toc" id=contents>Table of contents</h2>
<!--begin-toc-->
<ul class=toc>
<li><a href="#intro"><span class=secno>1. </span> Introduction</a>
<ul class=toc>
<li><a href="#placement"><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
Variables With Variable Properties</a>
<li><a href="#using-variables"><span class=secno>3. </span> Using
Variables: the var() function</a>
<ul class=toc>
<li><a href="#using-invalid-variables"><span class=secno>3.1. </span>
Using Invalid Variables</a>
</ul>
<li><a href="#cssom"><span class=secno>4. </span> CSSOM</a>
<ul class=toc>
<li><a href="#cssom-cssvariable"><span class=secno>4.1. </span>
Interface CSSVariableComponentValue</a>
<li><a href="#cssstyledeclaration-interface"><span class=secno>4.2.
</span> Additions to the <code>CSSStyleDeclaration</code> Interface</a>
</ul>
<li><a href="#conformance"><span class=secno>5. </span> Conformance</a>
<ul class=toc>
<li><a href="#conventions"><span class=secno>5.1. </span> Document
Conventions</a>
<li><a href="#conformance-classes"><span class=secno>5.2. </span>
Conformance Classes</a>
<li><a href="#partial"><span class=secno>5.3. </span> Partial
Implementations</a>
<li><a href="#experimental"><span class=secno>5.4. </span> Experimental
Implementations</a>
<li><a href="#testing"><span class=secno>5.5. </span>Non-Experimental
Implementations</a>
<li><a href="#cr-exit-criteria"><span class=secno>5.6. </span> CR Exit
Criteria</a>
</ul>
<li class=no-num><a href="#acknowledgments">Acknowledgments</a>
<li class=no-num><a href="#references">References</a>
<ul class=toc>
<li class=no-num><a href="#normative-references">Normative
references</a>
<li class=no-num><a href="#other-references">Other references</a>
</ul>
<li class=no-num><a href="#index">Index</a>
<li class=no-num><a href="#property-index">Property index</a>
</ul>
<!--end-toc-->
<h2 id=intro><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 user-defined properties known
collectively as <a href="#variable-property"><i>variable
properties</i></a>, which allow an author to assign arbitrary values to a
property with an author-chosen name, and 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
editting such files much easier and less error-prone, as one only has to
change the value once, at the variable definition site, and the change
will propagate to all uses of that variable automatically.
<h3 id=placement><span class=secno>1.1. </span> Module Interactions</h3>
<p>This module defines a new type of primitive value, the <a
href="#type-variable"><i>variable</i></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 Variables
With Variable Properties</h2>
<p>This specification defines an open-ended set of properties called <a
href="#variable-property"><i>variable properties</i></a>, which are used
to define <a href="#type-variable"><i>variables</i></a>.
<table class=propdef>
<tbody>
<tr>
<th>Name:
<td><dfn id=variable-property
title="variable property|variable properties">var-*</dfn>
<tr>
<th>Values:
<td>see prose
<tr>
<th>Initial:
<td>invalid (a keyword, but 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><em>Any</em> property name starting with the prefix "var-" is a <a
href="#variable-property"><i>variable property</i></a>. Variable
properties are defined to be valid but meaningless as they are meant
solely for allowing authors to pass custom data around their page, similar
to the <a
href="http://www.whatwg.org/specs/web-apps/current-work/multipage/elements.html#embedding-custom-non-visible-var-with-the-var-*-attributes">custom
data attributes</a> in HTML. Other specifications and user agents must not
assign a particular meaning to <a href="#variable-property"><i>variable
properties</i></a> or attach a specific effect to them beyond the bare
minimum that comes from them being valid properties.
<p class=issue>As defined here, the syntax for variable usage is different
from the syntax for variable definition (i.e. var-foo for definition,
var(foo) for usage). Some have suggested that the syntaxes should should
match, using functional syntax in both cases. Others have suggested using
a prefixed symbol instead of functional syntax (e.g. $foo) for both the
property and usage.
<p>The valid possible values of a variable property are almost completely
unrestricted. A variable property can contain anything that is valid
according to the <code>value</code> production in the <a
href="http://www.w3.org/TR/CSS2/syndata.html#tokenization">CSS Core
Grammar</a>. The values do not have to correspond to any existing CSS
values, as they are not evaluated except to replace variables occuring
within them until they are actually referenced in a normal property with a
variable.
<p>This specification reserves the use of all function tokens starting with
the prefix "var" within variable properties. Authors must not use any such
functions except as defined in this specification or future updates. If a
variable property contains such a function, it must match the grammar
defined in this specification or future updates; the use of such a
function that does not follow the grammar, or that utilizes such a
function that is not yet defined, makes the variable property invalid and
it must be ignored.
<div class=example>
<p>For example, even though this spec doesn't define a ‘<code
class=css>var-inherit()</code>’ function, that name is reserved by
default and cannot be used. Similarly, the ‘<code
class=css>var()</code>’ function <em>is</em> defined by this spec,
so any use of it must match the definition in the spec. The following
property declarations are thus invalid:</p>
<pre>
:root {
var-foo: var-inherit(bar); /* Use of reserved, but undefined, function. */
var-foo: var(bar, baz, qux); /* Invalid use of var() function. */
}</pre>
</div>
<p>For each <a href="#variable-property"><i>variable property</i></a>,
there is an associated variable with the same name save for the "var-"
prefix. For example, a variable property named "var-foo" is associated
with the variable named "foo". See the next chapter for details on how to
use variables.
<div class=example>
<p>This style rule:</p>
<pre>
:root {
var-header-color: #06c;
}</pre>
<p>declares a <a href="#variable-property"><i>variable property</i></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.</p>
</div>
<p>Variable 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>
<p>If a <a href="#variable-property"><i>variable property</i></a> is
declared multiple times, the standard cascade rules help resolve it.
Variables always draw from the computed value of the associated variable
property on the same element:</p>
<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="#variable-property"><i>Variable properties</i></a> may use
variables in their own values to build up composite variables. This can
create cyclic dependencies where two or more <a
href="#variable-property"><i>variable properties</i></a> each attempt to
use the variable that the other defines; doing so makes all the <a
href="#variable-property"><i>variable properties</i></a> involved in the
cycle compute to their initial value (which is a guaranteed-invalid
value).
<div class=example>
<p>This example shows a variable property safely using a variable:</p>
<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 will automatically update when the ‘<code
class=property>var-main-color</code>’ property is changed.</p>
</div>
<div class="example invalid-example">
<p>On the other hand, this example shows an invalid instance of variables
depending on each other:</p>
<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-variable"><i>invalid variables</i></a> rather than
lengths.</p>
</div>
<p>It is important to note that <a href="#variable-property"><i>variable
properties</i></a> resolve any <a
href="#type-variable"><i>variables</i></a> in their values at
computed-value time, which occurs <em>before</em> the value is inherited.
In general, cyclic dependencies occur only when multiple variable
properties on the same element refer to each other; variable 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>
<p>For example, given the following structure, these variable properties
are <strong>not</strong> cyclic, and all define valid variables:</p>
<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>’.</p>
</div>
<p>The initial value of a <a href="#variable-property"><i>variable
property</i></a> is a special invalid value which makes the associated
variable an <a href="#invalid-variable"><i>invalid variable</i></a>. This
is represented by the keyword ‘<code
class=css>invalid</code>’, but that keyword has no special meaning
in itself, and is valid if set explicitly in a variable property.
<div class=example>
<pre>
<div>
<p>Sample text</p>
</div>
<style>
p { var-foo: invalid; }
div,p { font-family: var(foo); }
</style></pre>
<p>In this example, the "foo" variable is an <a
href="#invalid-variable"><i>invalid variable</i></a> at the time the DIV
element references it, because the ‘<code
class=property>var-foo</code>’ property still has its initial
value. This causes the DIV's ‘<code
class=property>font-family</code>’ property to compute to the
initial value for ‘<code class=property>font-family</code>’.</p>
<p>On the other hand, the P element defines an explicit value for the
‘<code class=property>var-foo</code>’ property. Its
‘<code class=property>font-family</code>’ property thus
references a font named "invalid".</p>
</div>
<h2 id=using-variables><span class=secno>3. </span> Using Variables: the
var() function</h2>
<p>A <dfn id=variable>variable</dfn> allows the value of a <a
href="#variable-property"><i>variable property</i></a> on an element to be
substituted into another property on the element. Variables are used with
the ‘<code class=css>var()</code>’ function, taking the name
of a variable and returning the value of the associated variable property.
The syntax of the ‘<code class=css>var()</code>’ function is:
<pre><dfn id=type-variable
title="variable|variables"><variable></dfn> = var( IDENT )</pre>
<p>The ‘<code class=css>var()</code>’ function references the
<a href="#variable-property"><i>variable property</i></a> with the name
obtained by prepending "var-" to the function's argument. <span
class=note>Note that all possible <a href="#variable-property"><i>variable
properties</i></a> exist; if one with the given name has not yet been
defined in the document, it simply takes its initial value.</span> When a
property resolves its computed value, a ‘<code
class=css>var()</code>’ function is replaced by the value of the
referenced <a href="#variable-property"><i>variable property</i></a>.
<p>A variable can be used anywhere a value is expected in CSS. Variables
can not be used as property names, selectors, or anything else besides
property values - doing so either produces an invalid value or, in some
situations like the attribute value of an attribute selector, a valid
value that nonetheless has no relation to the variable of that name.
<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"><i>invalid at computed-value
time</i></a>.
<div class=example>
<p>For example, the following usage is fine from a syntax standpoint, but
results in nonsense when the variable is substituted in:</p>
<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>
</div>
<h3 id=using-invalid-variables><span class=secno>3.1. </span> Using Invalid
Variables</h3>
<p>When a variable property has its initial value, a ‘<code
class=css>var()</code>’ function referencing that property
represents an <dfn id=invalid-variable
title="invalid variable|invalid variables">invalid variable</dfn>. Using
an <a href="#invalid-variable"><i>invalid variable</i></a> in a property
value makes the declaration <a
href="#invalid-at-computed-value-time"><i>invalid at computed-value
time</i></a>.
<p>A declaration that is <dfn id=invalid-at-computed-value-time>invalid at
computed-value time</dfn> results from either using an <a
href="#invalid-variable"><i>invalid variable</i></a> in a property value,
or using a valid variable that produces an invalid declaration when it is
substituted in. When this happens, the declaration must compute to the
property's initial value.
<div class=example>
<p>For example, in the following code:</p>
<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>
<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.</p>
</div>
<p class=note>The <a href="#invalid-at-computed-value-time"><i>invalid at
computed-value time</i></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. I think ‘<code class=css>attr()</code>’ needs
to rely on it as well, as its behavior is almost identical to variables.
<h2 id=cssom><span class=secno>4. </span> CSSOM</h2>
<p><a href="#variable-property"><i>Variable properties</i></a> are ordinary
properties, and can be read or modified using all of the existing CSSOM
APIs for reading or modifying properties.
<p>The specification extends the IDL definitions in the CSSOM spec to
represent the use of the ‘<code class=css>var()</code>’
function.
<h3 id=cssom-cssvariable><span class=secno>4.1. </span> Interface
CSSVariableComponentValue</h3>
<p>The CSSVariableComponentValue interface represents a use of the
‘<code class=css>var()</code>’ function.
<dl>
<dt>IDL Definition
<dd>
<pre class=idl>
[NoInterfaceObject] interface CSSVariableComponentValue {
attribute DOMString variableName;
readonly attribute any variableValue;
}</pre>
<dt>Attributes
<dd>
<dl>
<dt><code>variableName</code> of type <code>DOMString</code>
<dd>This attribute represents the argument to the function. Changing
this attribute changes the <a href="#variable-property"><i>variable
property</i></a> being referred to.
<dt><code>variableValue</code> of type <code>any</code>, readonly
<dd>This attribute represents the value of the <a
href="#variable-property"><i>variable property</i></a> the function is
referencing.
</dl>
</dl>
<h3 id=cssstyledeclaration-interface><span class=secno>4.2. </span>
Additions to the <code>CSSStyleDeclaration</code> Interface</h3>
<p class=issue>The set of variable properties is open-ended, so it's not
clear how best to represent this. Ideally, the CSSOM would expose the
current set of properties with a non-initial value and allow setting of
arbitrary properties. The most natural way seems to be to first, set up a
getter behavior on the interface somehow that deals with variable
properties, and second, set up a <code>vars</code> map that exposes the
variable properties that aren't set to their initial value.
<h2 id=conformance><span class=secno>5. </span> Conformance</h2>
<h3 id=conventions><span class=secno>5.1. </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.</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.
<h3 id=conformance-classes><span class=secno>5.2. </span> Conformance
Classes</h3>
<p>Conformance to CSS Variables Module 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 CSS Variables Module if all of its
declarations that use properties defined in this module have values that
are valid according to the generic CSS grammar and the individual grammars
of each property as given in this module.
<p>A renderer is conformant to CSS Variables Module if, in addition to
interpreting the style sheet as defined by the appropriate specifications,
it supports all the features defined by CSS Variables Module 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 CSS Variables Module 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 id=partial><span class=secno>5.3. </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 id=experimental><span class=secno>5.4. </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 id=testing><span class=secno>5.5. </span>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.
<h3 id=cr-exit-criteria><span class=secno>5.6. </span> CR Exit Criteria</h3>
<p> For this specification to be advanced to Proposed Recommendation, there
must be at least two independent, interoperable implementations of each
feature. Each feature may be implemented by a different set of products,
there is no requirement that all features be implemented by a single
product. For the purposes of this criterion, we define the following
terms:
<dl>
<dt>independent
<dd>each implementation must be developed by a different party and cannot
share, reuse, or derive from code used by another qualifying
implementation. Sections of code that have no bearing on the
implementation of this specification are exempt from this requirement.
<dt>interoperable
<dd>passing the respective test case(s) in the official CSS test suite,
or, if the implementation is not a Web browser, an equivalent test. Every
relevant test in the test suite should have an equivalent test created if
such a user agent (UA) is to be used to claim interoperability. In
addition if such a UA is to be used to claim interoperability, then there
must one or more additional UAs which can also pass those equivalent
tests in the same way for the purpose of interoperability. The equivalent
tests must be made publicly available for the purposes of peer review.
<dt>implementation
<dd>a user agent which:
<ol class=inline>
<li>implements the specification.
<li>is available to the general public. The implementation may be a
shipping product or other publicly available version (i.e., beta
version, preview release, or “nightly build”). Non-shipping product
releases must have implemented the feature(s) for a period of at least
one month in order to demonstrate stability.
<li>is not experimental (i.e., a version specifically designed to pass
the test suite and is not intended for normal usage going forward).
</ol>
</dl>
<p>The specification will remain Candidate Recommendation for at least six
months.
<h2 class=no-num id=acknowledgments>Acknowledgments</h2>
<p>Thanks to Daniel Glazman and Dave Hyatt for writing the original
Variables draft in 2008. Thanks to many WG members for keeping the idea of
variables alive through the years. Thanks to Roland Steiner and Shane
Stephens for invaluable feedback and implementation experience.
<h2 class=no-num id=references>References</h2>
<h3 class=no-num id=normative-references>Normative references</h3>
<!--begin-normative-->
<!-- Sorted by label -->
<dl class=bibliography>
<dt style="display: none"><!-- keeps the doc valid if the DL is empty -->
<!---->
<dt id=CSS21>[CSS21]
<dd>Bert Bos; et al. <a
href="http://www.w3.org/TR/2011/REC-CSS2-20110607"><cite>Cascading Style
Sheets Level 2 Revision 1 (CSS 2.1) Specification.</cite></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>[RFC2119]
<dd>S. Bradner. <a href="http://www.ietf.org/rfc/rfc2119.txt"><cite>Key
words for use in RFCs to Indicate Requirement Levels.</cite></a> Internet
RFC 2119. URL: <a
href="http://www.ietf.org/rfc/rfc2119.txt">http://www.ietf.org/rfc/rfc2119.txt</a>
</dd>
<!---->
</dl>
<!--end-normative-->
<h3 class=no-num id=other-references>Other references</h3>
<!--begin-informative-->
<!-- Sorted by label -->
<dl class=bibliography>
<dt style="display: none"><!-- keeps the doc valid if the DL is empty -->
<!---->
<dt id=CSS1>[CSS1]
<dd>Håkon Wium Lie; Bert Bos. <a
href="http://www.w3.org/TR/2008/REC-CSS1-20080411"><cite>Cascading Style
Sheets (CSS1) Level 1 Specification.</cite></a> 11 April 2008. W3C
Recommendation. URL: <a
href="http://www.w3.org/TR/2008/REC-CSS1-20080411">http://www.w3.org/TR/2008/REC-CSS1-20080411</a>
</dd>
<!---->
<dt id=CSS3COLOR>[CSS3COLOR]
<dd>Tantek Çelik; Chris Lilley; L. David Baron. <a
href="http://www.w3.org/TR/2011/REC-css3-color-20110607"><cite>CSS Color
Module Level 3.</cite></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>
<!--end-informative-->
<h2 class=no-num id=index>Index</h2>
<!--begin-index-->
<ul class=indexlist>
<li>authoring tool, <a href="#authoring-tool"
title="authoring tool"><strong>5.2.</strong></a>
<li>invalid at computed-value time, <a
href="#invalid-at-computed-value-time"
title="invalid at computed-value time"><strong>3.1.</strong></a>
<li>invalid variable, <a href="#invalid-variable"
title="invalid variable"><strong>3.1.</strong></a>
<li>invalid variables, <a href="#invalid-variable"
title="invalid variables"><strong>3.1.</strong></a>
<li>renderer, <a href="#renderer" title=renderer><strong>5.2.</strong></a>
<li>style sheet
<ul>
<li>as conformance class, <a href="#style-sheet"
title="style sheet, as conformance class"><strong>5.2.</strong></a>
</ul>
<li>variable, <a href="#type-variable"
title=variable><strong>3.</strong></a>, <a href="#variable"
title=variable><strong>3.</strong></a>
<li>variable properties, <a href="#variable-property"
title="variable properties"><strong>2.</strong></a>
<li>variable property, <a href="#variable-property"
title="variable property"><strong>2.</strong></a>
<li>variables, <a href="#type-variable"
title=variables><strong>3.</strong></a>
</ul>
<!--end-index-->
<h2 class=no-num id=property-index>Property index</h2>
<!--begin-properties-->
<table class=proptable>
<thead>
<tr>
<th>Property
<th>Values
<th>Initial
<th>Applies to
<th>Inh.
<th>Percentages
<th>Media
<tbody>
<tr>
<th><span class=property>var-*</span>
<td>see prose
<td>invalid (a keyword, but see prose)
<td>all elements
<td>yes
<td>specified value with variables substituted (but see prose for
"invalid variables")
<td>all
</table>
<!--end-properties-->
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