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<!DOCTYPE html>
<html lang="en">
<head profile="http://www.w3.org/2006/03/hcard">
<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/css-module/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="[VERSION]">http://dev.w3.org/csswg/css-variables/</a>
<!--<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/</var></a>
<dt>Editors:
<dd class=vcard>
<a href="http://xanthir.com/contact/" rel=url>
<span class=fn>Tab Atkins Jr.</span>,
</a>
<span class=org>Google, Inc.</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 <b>Variables</b>, which allow a value to be assigned to a name, which may then be used in place of the value 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 <b>Variable</b>, 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</h2>
<p>A ''@var'' rule defines a variable and assigns a value to it. It consists of the at-keyword ''@var'' followed by a variable name (an identifier starting with the "$" character), followed by the value and finally terminated with a semi-colon. The result of this rule is that the given value is assigned to the given variable name.<p>
<p class='note'>I chose the $ prefix over the var() function for terseness. We should try to avoid nesting functions as much as reasonable, as it's hard to read. The $ is required in the variable definition as well as the variable use for consistency - every instance of the variable name within CSS has the same form.</p>
<div class=example>
<p>This variables declaration:</p>
<pre>@var $header-color #06c;</pre>
<p>declares a variable named "$header-color", and assigns to it the value "#06c". This can then be used in any place where that value would be valid, such as:
<pre>h1 { background-color: $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 $header-color is used elsewhere in the stylesheet, all of the uses can be updated at once by changing the variable declaration.</p>
</div>
<p>Defined variables are available to all stylesheets in the document. Thus, using ''@import'' to include a stylesheet also includes all variables defined in the stylesheet (and makes all variables declared outside available within the stylesheet, if applicable). ''@var'' rules present in disabled or alternate stylesheets do not define variables. Scoped stylesheets are an exception to this: variables defined in the global scope are available within a scoped stylesheet, but variables defined or imported within scoped stylesheets are only available within the scoped stylesheet and any imported stylesheets.</p>
<p class='note'>Making all variables global is the simplest solution that works, and is consistent with other at-rules that define names, like @font-face and @counter-style - the web seems to get by great on "ad-hoc scoping" via adding short prefixes. If it is shown that we need a more complex scoping solution in the future, I believe we can do so via a general scoping/namespacing mechanism that applies to all the at-rules that define names.</p>
<p>A ''@var'' rule is <dfn id='inactive-var-rule'>inactive</dfn> if it's present in a disabled or alternate stylesheet, a stylesheet with a media query that evaluates to false, or is present within a conditional rule group [[CSS3CONDITIONAL]] whose condition evaluates to false. <i>Inactive</i> ''@var'' rules do not define variables. All other ''@var'' rules are <dfn id='active-var-rule'>active</dfn>.</p>
<p>If new ''@var'' rules are added to the document through any means, such as direct CSSOM manipulation or new stylesheets being added to the document, or if an <i>inactive</i> ''@var'' rule becomes <i>active</i>, they define variables. Similarly, if ''@var'' rules are removed from the document or become <i>inactive</i>, they no longer define variables.</p>
<p>If multiple <i>active</i> ''@var'' rules attempt to define variables with the same name, the last one defined wins; the preceding ''@var'' rules do not define a variable. For this purpose, user-agent defined variable rules come before author-defined variable rules, which come before user-defined variable rules, which come before override variable rules. Within each of the preceding groups, the ordering is document order.</p>
<div class='example'>
<p>Earlier declarations of the same variable have no effect, even if uses of the variable come between the earlier and final attempts to define the variable:</p>
<pre>
@var $var red;
p { color: $var; }
@var $var blue;</pre>
<p>In this example, the color of <p> elements is blue, because the second declaration wins over the first declaration in all uses.</p>
</div>
<p>Variables can refer to other variables in their value. If a dependency cycle is created, all the declarations that directly contribute to the cycle define <i>invalid variables</i>.</p>
<div class='example'>
<p>For example, the following set of variable declarations create a dependency cycle:</p>
<pre>
@var $foo red;
@var $bar linear-gradient(transparent, $foo);
@var $foo $bar;</pre>
<p>The last two declarations create the cycle, and so they define ''$bar'' and ''$foo'', respectively, as <i>invalid variables</i> instead of as the values in the declarations. If the last rule were removed, however, there would be no problem - ''$foo'' would be defined as ''red'', while ''$bar'' would be defined as ''linear-gradient(transparent, red)''.</p>
</div>
<p>Before they are defined, all variables are <i>invalid variables</i>.</p>
<h2 id='using-variables'>
Using Variables</h2>
<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>
@var $looks-valid 20px;
p { background-color: $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>An <dfn title="invalid variable|invalid variables">invalid variable</dfn> results from having variables directly or indirectly refer to themselves, or from using an undefined variable. 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>
@var $invalid $invalid; /* Invalid, due to the trivial cycle. */
p { background-color: red; }
p { background-color: $invalid; }</pre>
<p>the <p> elements will have transparent backgrounds (the initial value for 'background-color'), rather than red backgrounds.</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'>
APIs</h2>
<p>CSS Variables are mutable - one can change them after they've been defined, through the CSSOM. This can be done in two ways: one can read the current variable definition and set an override definition through the convenient <code>.vars</code> property, or manipulate the definitions in the stylesheets directly through the standard CSSOM stylesheet interface.</p>
<h3 id='cssom-simple'>
The Simple API</h3>
<p class='issue'>Not exactly sure how to define this, but I want to put a 'css' property on <code>document</code> (and hopefully <code>window</code>) as a hook for this and other future CSS apis. <code>document.css</code> would then expose a 'vars' property that implements CSSOverrideVariablesMap. Variables are document-global, so they need to be exposed at the document level, not the stylesheet level.</p>
<p class='issue'>Do I need to define the concept of the override stylesheet? Several browsers already expose this concept of a script-only stylesheet that overrides all other stylesheets.</p>
<p>The CSSOverrideVariablesMap presents a simple interface to the variables in the override stylesheet, which take precedence over variables defined elsewhere in the document. As a convenience, it also allows reading normal variables defined elsewhere in the document, as long as there is no variable with the same name in the override stylesheet.</p>
<pre class='idl'>
[NoInterfaceObject] CSSOverrideVariablesMap {
getter any (DOMString variableName);
setter void (DOMString variableName, any variableValue);
creator void (DOMString variableName, any variableValue);
deleter void (DOMString variableName);
}</pre>
<p>On getting, if a variable named <var>variableName</var> exists in the override stylesheet, return its value. Otherwise, if a variable named <var>variableName</var> is defined in the document, return its value. Otherwise, return null.</p>
<p>On setting, find the variable named <var>variableName</var> in the override stylesheet and set its value to <var>variableValue</var>.</p>
<p>On creating, append a new variable rule to the override stylesheet, with the variable name set to <var>variableName</var> and the value set to <var>variableValue</var>.</p>
<p>On deleting, remove the variable with the name <var>variableName</var> from the override stylesheet.</p>
<h3 id='cssom-stylesheet'>
Additions to the Stylesheet Interface</h3>
<p>This specification extends the IDL definitions in the CSSOM spec [[!CSSOM]] in several ways.</p>
<h4 id='cssom-cssrule'>
Changes to interface CSSRule</h4>
<dl>
<dt>IDL Definition</dt>
<dd>
<pre class='idl'>
interface CSSRule {
// RuleType
const unsigned short UNKNOWN_RULE = 0;
const unsigned short STYLE_RULE = 1;
const unsigned short CHARSET_RULE = 2;
const unsigned short IMPORT_RULE = 3;
const unsigned short MEDIA_RULE = 4;
const unsigned short FONT_FACE_RULE = 5;
const unsigned short PAGE_RULE = 6;
<ins>const unsigned short <code>VARIABLE_RULE</code> = 11;</ins>
readonly attribute unsigned short type;
attribute DOMString cssText;
// raises(DOMException) on setting
readonly attribute CSSStyleSheet parentStyleSheet;
readonly attribute CSSRule parentRule;
};</pre>
<dt>Defined Constants</dt>
<dd><dfn><code>VARIABLE_RULE</code></dfn>: The rule is a <code>CSSVariableRule</code>.</dd>
</dl>
<p class='issue'>Going with value 11 for now, since CSSOM seems to reserve 0-10.</p>
<h4 id='cssom-cssvariablerule'>
Interface CSSVariableRule</h4>
<p>The <code>CSSVariableRule</code> interface represents a ''@var'' rule within a CSS stylesheet. The ''@var'' rule is used to define variables.</p>
<dl>
<dt>IDL Definition</dt>
<dd>
<pre class='idl'>
interface CSSVariableRule : CSSRule {
attribute DOMString name;
attribute DOMString value;
}</pre>
</dd>
</dl>
<h4 id='cssom-cssvariable'>
Interface CSSVariableComponentValue</h4>
<p>The CSSVariableComponentValue interface represents a call to a CSS Variable.</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 is used for the name of the variable. Changing this attribute changes the variable being referred to.</dd>
<dt><code>variableValue</code> of type <code>any</code>, readonly</dt>
<dd>This attribute is used for the value of the variable.</dd>
</dl>
</dd>
</dl>
<h2 id='grammar'>
The Grammar of Variables</h2>
<p class='issue'>I'm not sure if I've done this section correctly. For now, I'll try my best to copypasta what Conditionals is doing, since dbaron usually know what's what.</p>
<p>This specification extends the lexical scanner in the <a href="http://www.w3.org/TR/CSS21/grammar.html">Grammar of CSS 2.1</a> (<a href="#CSS21" rel="biblioentry">[CSS21]<!--{{!CSS21}}--></a>, Appendix G) by adding:</p>
<pre>
@{V}{A}{R} {return VARIABLE_SYM;}
"$"name {return VARIABLE;}</pre>
<p>and the grammar by adding:</p>
<pre>
variable_rule
: VARIABLE_SYM S+ variable_name S+ expr ':';
;</pre>
<p>and by amending:</p>
<pre>
stylesheet
: [ CHARSET_SYM STRING ';' ]?
[S|CDO|CDC]* [ import [ CDO S* | CDC S* ]* ]*
[ [ ruleset | media | page <ins>| variable_rule</ins> ] [ CDO S* | CDC S* ]* ]*
;
nested_statement
: ruleset | media | page | font_face_rule | keyframes-rule |
supports_rule | document_rule <ins>| variable_rule</ins>
;
term
: unary_operator?
[ NUMBER S* | PERCENTAGE S* | LENGTH S* | EMS S* | EXS S* | ANGLE S* |
TIME S* | FREQ S* ]
| STRING S* | IDENT S* | URI S* | hexcolor | function <ins>| VARIABLE S*</ins>
;</pre>
<p class='issue'>This uses the 'nested_statement' production from Conditionals. That should make its way to a proper draft, like a new release of Syntax.</p>
<p class='issue'>Should variables be usable elsewhere, like in the value of a MQ?</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 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
ignore unsupported component values and honor supported values in a single
multi-value property declaration: if any value is considered invalid
(as unsupported values must be), CSS requires that the entire declaration
be ignored.</p>
<h3 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 class=issue>[Change or remove the following CR exit criteria if
the spec is not a module, but, e.g., a Note or a profile. This text was <a
href="http://www.w3.org/Style/CSS/Tracker/actions/44"> decided on 2008-06-04.</a>]</p>
<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.</p>
<h2 class=no-num id="references">References</h2>
<h3 class="no-num" id="normative-references">Normative references</h3>
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<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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