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<!DOCTYPE html>
<html lang="en">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
<title>CSS Variables Module Level 1</title>
<link rel="stylesheet" type="text/css" href="http://dev.w3.org/csswg/default.css">
<link rel="stylesheet" type="text/css" href="http://www.w3.org/StyleSheets/TR/W3C-[STATUS].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>
</head>
<div class="head">
<!--logo-->
<h1>CSS Variables Module Level 1</h1>
<h2 class="no-num no-toc">[LONGSTATUS] [DATE]</h2>
<dl>
<dt>This version:
<!--<dd><a href="http://www.w3.org/TR/[YEAR]/WD-css-variables-[CDATE]/">[VERSION]</a>-->
<dd><a href="http://dev.w3.org/csswg/[SHORTNAME]/">Editor's draft</a> (version of [DATE])
<dt>Latest version:
<dd><a href="http://www.w3.org/TR/[SHORTNAME]/">http://www.w3.org/TR/[SHORTNAME]/</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 href="http://xanthir.com/contact/" class='url'>
<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>
<!--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 [[!CSS21]], which builds on CSS level 1 [[CSS1]]. 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>
<!--status-->
<!--<p>The following features are at risk: …</p>-->
<h2 class="no-num no-toc" id="contents">Table of contents</h2>
<!--toc-->
<h2 id="intro">
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 user-defined properties known collectively as <i>variable properties</i>, 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.</p>
<h3 id="placement">
Module Interactions</h3>
<p>This module defines a new type of primitive value, the <i>variable</i>, which is accepted by all properties.</p>
<h3 id="values">
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 [[!CSS21]]. Value types not defined in
this specification are defined in CSS Level 2 Revision 1 [[!CSS21]].
Other CSS modules may expand the definitions of these value types: for
example [[CSS3COLOR]], when combined with this module, expands the
definition of the <color> value type as used in this specification.</p>
<h2 id="defining-variables">
Defining Variables With Variable Properties</h2>
<p>This specification defines an open-ended set of properties called <i>variable properties</i>, which are used to define <i>variables</i>.</p>
<table class='propdef'>
<tr>
<th>Name:</th>
<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 <i>variable property</i>. 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.w3.org/TR/html5/global-attributes.html#embedding-custom-non-visible-data-with-the-data-attributes">custom data attributes</a> in HTML. Other specifications and user agents must not assign a particular meaning to <i>variable properties</i> or attach a specific effect to them beyond the bare minimum that comes from them being valid properties.</p>
<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>
<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>
<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.</p>
<div class='example'>
<p>For example, even though this spec doesn't define a ''var-inherit()'' function, that name is reserved by default and cannot be used. Similarly, the ''var()'' 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 <i>variable property</i>, 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.</p>
<div class=example>
<p>This style rule:</p>
<pre>
:root {
var-header-color: #06c;
}</pre>
<p>declares a <i>variable property</i> 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 ''background-color: #06c;'', except that the variable name makes the origin of the color clearer, and if ''var(header-color)'' is used on other elements in the document, all of the uses can be updated at once by changing the 'var-header-color' 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 ''@media'' and other conditional rules, can be used in HTML's <code>style</code> att
F872
ribute, can be read or set using the CSSOM, etc..</p>
<div class='example'>
<p>If a <i>variable property</i> 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><i>Variable properties</i> may use variables in their own values to build up composite variables. This can create cyclic dependencies where two or more <i>variable properties</i> each attempt to use the variable that the other defines; doing so makes all the <i>variable properties</i> involved in the cycle compute to their initial value (which is a guaranteed-invalid value).</p>
<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 'var-accent-background' property will automatically update when the 'var-main-color' 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 'var-one' and 'var-two' now define <i>invalid variables</i> rather than lengths.</p>
</div>
<p>It is important to note that <i>variable properties</i> resolve any <i>variables</i> 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.</p>
<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 'var-foo'. The <two> element inherits this value, and additionally assigns a value to 'var-bar' using the ''foo'' variable. Finally, the <three> element inherits the 'var-bar' value <em>after</em> variable substitution (in other words, it sees the value ''calc(10px + 10px)''), and then redefines 'var-foo' in terms of that value. Since the value it inherited for 'var-bar' no longer contains a reference to the 'var-foo' property defined on <one>, defining 'var-foo' using the ''var(bar)'' variable is not cyclic, and actually defines a value that will eventually (when referenced as a variable in a normal property) resolve to ''30px''.</p>
</div>
<p>The initial value of a <i>variable property</i> is a special invalid value which makes the associated variable an <i>invalid variable</i>. This is represented by the keyword ''invalid'', but that keyword has no special meaning in itself, and is valid if set explicitly in a variable property.</p>
<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 <i>invalid variable</i> at the time the DIV element references it, because the 'var-foo' property still has its initial value. This causes the DIV's 'font-family' property to compute to the initial value for 'font-family'.</p>
<p>On the other hand, the P element defines an explicit value for the 'var-foo' property. Its 'font-family' property thus references a font named "invalid".</p>
</div>
<h2 id='using-variables'>
Using Variables: the var() function</h2>
<p>A <dfn>variable</dfn> allows the value of a <i>variable property</i> on an element to be substituted into another property on the element. Variables are used with the ''var()'' function, taking the name of a variable and returning the value of the associated variable property. The syntax of the ''var()'' function is:</p>
<pre><dfn id='type-variable' title="variable|variables"><variable></dfn> = var( IDENT )</pre>
<p>The ''var()'' function references the <i>variable property</i> with the name obtained by prepending "var-" to the function's argument. <span class='note'>Note that all possible <i>variable properties</i> 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 ''var()'' function is replaced by the value of the referenced <i>variable property</i>.</p>
<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>
<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 <i>invalid at computed-value time</i>.</p>
<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 ''20px'' is an invalid value for 'background-color', this instance of the property computes to 'transparent' (the initial value for 'background-color') instead.</p>
</div>
<h3 id='using-invalid-variables'>
Using Invalid Variables</h3>
<p>When a variable property has its initial value, a ''var()'' function referencing that property represents an <dfn title="invalid variable|invalid variables">invalid variable</dfn>. Using an <i>invalid variable</i> in a property value makes the declaration <i>invalid at computed-value time</i>.</p>
<p>A declaration that is <dfn>invalid at computed-value time</dfn> results from either using an <i>invalid variable</i> 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.</p>
<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 'background-color'), 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 ''background-color: 20px'' directly in their stylesheet - that would be a normal syntax error, which would cause the rule to be discarded, so the ''background-color: red'' rule would be used instead.</p>
</div>
<p class='note'>The <i>invalid at computed-value time</i> 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 ''attr()'' needs to rely on it as well, as its behavior is almost identical to variables.</p>
<h2 id='cssom'>
CSSOM</h2>
<p><i>Variable properties</i> are ordinary properties, and can be read or modified using all of the existing CSSOM APIs for reading or modifying properties.</p>
<p>The specification extends the IDL definitions in the CSSOM spec to represent the use of the ''var()'' function.</p>
<h3 id='cssom-cssvariable'>
Interface CSSVariableComponentValue</h3>
<p>The CSSVariableComponentValue interface represents a use of the ''var()'' function.</p>
<dl>
<dt>IDL Definition</dt>
<dd>
<pre class='idl'>
[NoInterfaceObject] interface CSSVariableComponentValue {
attribute DOMString variableName;
readonly attribute any variableValue;
}</pre>
</dd>
<dt>Attributes</dt>
<dd>
<dl>
<dt><code>variableName</code> of type <code>DOMString</code></dt>
<dd>This attribute represents the argument to the function. Changing this attribute changes the <i>variable property</i> being referred to.</dd>
<dt><code>variableValue</code> of type <code>any</code>, readonly</dt>
<dd>This attribute represents the value of the <i>variable property</i> the function is referencing.</dd>
</dl>
</dd>
</dl>
<h3 id='cssstyledeclaration-interface'>
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.</p>
<h2 id="conformance">
Conformance</h2>
<h3 id="conventions">
Document Conventions</h3>
<p>Conformance requirements are expressed with a combination of
descriptive assertions and RFC 2119 terminology. The key words “MUST”,
“MUST NOT”, “REQUIRED”, “SHALL”, “SHALL NOT”, “SHOULD”, “SHOULD NOT”,
“RECOMMENDED”, “MAY”, and “OPTIONAL” in the normative parts of this
document are to be interpreted as described in RFC 2119.
However, for readability, these words do not appear in all uppercase
letters in this specification.
<p>All of the text of this specification is normative except sections
explicitly marked as non-normative, examples, and notes. [[!RFC2119]]</p>
<p>Examples in this specification are introduced with the words “for example”
or are set apart from the normative text with <code>class="example"</code>,
like this:
<div class="example">
<p>This is an example of an informative example.</p>
</div>
<p>Informative notes begin with the word “Note” and are set apart from the
normative text with <code>class="note"</code>, like this:
<p class="note">Note, this is an informative note.</p>
<h3 id="conformance-classes">
Conformance Classes</h3>
<p>Conformance to CSS Variables Module
is defined for three conformance classes:
<dl>
<dt><dfn 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>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.
<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.
</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
134E
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">
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
ign
B90A
ore 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 id="experimental">
Experimental Implementations</h3>
<p>To avoid clashes with future CSS features, the CSS2.1 specification
reserves a <a href="http://www.w3.org/TR/CSS21/syndata.html#vendor-keywords">prefixed
syntax</a> for proprietary and experimental extensions to CSS.
<p>Prior to a specification reaching the Candidate Recommendation stage
in the W3C process, all implementations of a CSS feature are considered
experimental. The CSS Working Group recommends that implementations
use a vendor-prefixed syntax for such features, including those in
W3C Working Drafts. This avoids incompatibilities with future changes
in the draft.
</p>
<h3 id="testing">Non-Experimental Implementations</h3>
<p>Once a specification reaches the Candidate Recommendation stage,
non-experimental implementations are possible, and implementors should
release an unprefixed implementation of any CR-level feature they
can demonstrate to be correctly implemented according to spec.
<p>To establish and maintain the interoperability of CSS across
implementations, the CSS Working Group requests that non-experimental
CSS renderers submit an implementation report (and, if necessary, the
testcases used for that implementation report) to the W3C before
releasing an unprefixed implementation of any CSS features. Testcases
submitted to W3C are subject to review and correction by the CSS
Working Group.
<p>Further information on submitting testcases and implementation reports
can be found from on the CSS Working Group's website at
<a href="http://www.w3.org/Style/CSS/Test/">http://www.w3.org/Style/CSS/Test/</a>.
Questions should be directed to the
<a href="http://lists.w3.org/Archives/Public/public-css-testsuite">public-css-testsuite@w3.org</a>
mailing list.
<h3 id="cr-exit-criteria">
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.</p>
<h2 class=no-num id="references">References</h2>
<h3 class="no-num" id="normative-references">Normative references</h3>
<!--normative-->
<h3 class="no-num" id="other-references">Other references</h3>
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<h2 class="no-num" id="index">Index</h2>
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<h2 class="no-num" id="property-index">Property index</h2>
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