This module introduces cascading variables as a new primitive value type that is accepted by all CSS properties, and custom properties for defining them.
This section is not normative.
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.
This module introduces a family of custom author-defined properties known collectively as custom properties, which allow an author to assign arbitrary values to a property with an author-chosen name, and variables, 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 custom property, and the change will propagate to all uses of that variable automatically.
This module defines a new type of primitive value, the variable, which is accepted by all properties.
This specification follows the CSS property definition conventions 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.
This specification defines an open-ended set of properties called custom properties, which are used to define variables.
| Name: | var-* |
|---|---|
| Values: | [ <value> | <CDO> | <CDC> ]* |
| Initial: | (nothing, see prose) |
| Applies To: | all elements |
| Inherited: | yes |
| Computed Value: | specified value with variables substituted (but see prose for "invalid variables") |
| Media: | all |
A custom property is any property
whose name is composed of "var-" followed by an <ident> [[!CSS3VAL]]
Custom properties are solely for use by authors and users;
CSS will never give them a meaning beyond what is presented here.
Unlike other CSS properties, custom properties are case-sensitive. The "var-" prefix must be written in lower-case.
While both ''var-foo'' and ''var-FOO'' are valid, they are distinct properties - using ''var(foo)'' will refer to the first one, while using ''var(FOO)'' will refer to the second.
Custom properties have an extremely permissive value grammar. The <value> in its grammar corresponds to the "value" production in CSS 2.1 Chapter 4.1 [[!CSS21]], while <CDO> and <CDC> 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).
Note: Custom properties can contain a trailing ''!important'', but this is automatically removed from the property's value by the CSS parser, and makes the custom property "important" in the CSS cascade.
var-foo: if(x > 5) this.width = 10;
While this value is obviously useless as a variable, as it would be invalid in any normal property, it might be read and acted on by JavaScript.
There are an infinity of custom properties, but the initial value of a custom property is an empty invalid value. This means that, until a custom property is explicitly defined otherwise by a style sheet, it defines an invalid variable.
The primary purpose of custom properties is to define cascading variables. In CSS, a cascading variable 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 custom property defines a corresponding variable with the same name, minus the "var-" prefix. For example, the custom property 'var-foo' defines a variable named ''foo''. See the next chapter for details on how to use variables.
Custom properties 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.
This style rule:
:root {
var-header-color: #06c;
}
declares a custom property 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:
h1 { background-color: var(header-color); }
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.
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 ''@media'' and other conditional rules,
can be used in HTML's style attribute,
can be read or set using the CSSOM, etc..
If a custom property 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:
: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>
Custom properties may use variables in their own values to build up composite variables. This can create cyclic dependencies where two or more custom properties each attempt to use the variable that the other defines; doing so makes all the custom properties involved in the cycle compute to their initial value (which is a guaranteed-invalid value).
This example shows a custom property safely using a variable:
:root {
var-main-color: #c06;
var-accent-background: linear-gradient(to top, var(main-color), white);
}
The 'var-accent-background' property (along with any other properties that use ''var(main-color)'') will automatically update when the 'var-main-color' property is changed.
On the other hand, this example shows an invalid instance of variables depending on each other:
:root {
var-one: calc(var(two) + 20px);
var-two: calc(var(one) - 20px);
}
Both 'var-one' and 'var-two' now define invalid variables rather than lengths.
It is important to note that custom properties resolve any variables in their values at computed-value time, which occurs before 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.
For example, given the following structure, these custom properties are not cyclic, and all define valid variables:
<one><two><three /></two></one>
one { var-foo: 10px; }
two { var-bar: calc(var(foo) + 10px); }
three { var-foo: calc(var(bar) + 10px); }
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 after 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''.
Every custom property automatically defines a corresponding cascading variable, which can then be substituted into another property with the ''var()'' function. The syntax of ''var()'' is:
<variable> = var( variable-name [, <fallback> ]? )
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.)
The <fallback> value is identical to the syntax of a custom property.
If the variable named by the first argument is valid, the variable's value is substituted as normal. If it's invalid, and a <fallback> was provided, the <fallback> is substituted instead. Otherwise, the variable is an invalid variable.
For example, the following code incorrectly attempts to use a variable as a property name:
.foo {
var-side: margin-top;
var(side): 20px;
}
This is not equivalent to setting ''margin-top: 20px;''. Instead, the second declaration is simply thrown away as a syntax error for having an invalid property name.
Similarly, you can't build up a single token where part of it is provided by a variable:
.foo {
var-gap: 20;
margin-top: var(gap)px;
}
Again, this is not equivalent to setting ''margin-top: 20px;'' (a length). Instead, it's equivalent to ''margin-top: 20 px;'' (a number followed by an ident), which is simply an invalid value for the 'margin-top' property. Note, though, that ''calc()'' can be used to validly achieve the same thing, like so:
.foo {
var-gap: 20;
margin-top: calc(var(gap) * 1px);
}
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 invalid at computed-value time.
For example, the following usage is fine from a syntax standpoint, but results in nonsense when the variable is substituted in:
:root { var-looks-valid: 20px; }
p { background-color: var(looks-valid); }
Since ''20px'' is an invalid value for 'background-color', this instance of the property computes to 'transparent' (the initial value for 'background-color') instead.
If the property was one that's inherited by default, such as 'color', it would compute to the inherited value rather than the initial value.
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.
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.
When a custom property has its initial value, the variable it defines represents an invalid variable. Using an invalid variable in a property value (including other custom properties) makes the declaration invalid at computed-value time.
A declaration can be invalid at computed-value time if it uses an invalid variable, as explained above, or if it uses a valid variable, but the property value, after substituting its variables, 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.
For example, in the following code:
:root { var-not-a-color: 20px; }
p { background-color: red; }
p { background-color: var(not-a-color); }
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.
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.
The invalid at computed-value time 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.
CSSStyleDeclaration Interface
The CSSStyleDeclaration interface is amended as follows:
partial interface CSSStyleDeclaration {
attribute CSSVariablesDeclaration var;
}
Custom property names must be serialized exactly as provided by the author.
CSSVariablesDeclaration Interface
The CSSVariablesDeclaration interface exposes the custom properties declared in the parent declaration block,
in a more convenient fashion than the getPropertyValue()/etc. functions.
interface CSSVariablesDeclaration {
getter DOMString (DOMString varName);
setter creator void (DOMString varName, DOMString varValue);
deleter void (DOMString varName);
}
The supported property names on a CSSStyleDeclaration object are the property names of all the custom properties in the CSS declaration block declarations, with the "var-" prefix removed.
Before running any of the algorithms in this section, prepend "var-" to varName's value.
When asked to get the value of a variable,
if varName is in the CSS declaration block declarations,
invoke getPropertyValue() by passing varName as its argument,
and return the returned value.
Otherwise, return the empty string.
When asked to set or create the value of a variable,
if varName matches the grammar of a custom property name:
if varValue is the empty string,
invoke the algorithm to delete a variable;
otherwise, invoke setProperty() by passing varName as the property argument and varValue as the value argument.
When asked to delete the value of a variable,
if varName matches the grammar of a custom property name,
invoke removeProperty() by passing varName as its argument,
and return the returned value.
Otherwise, do nothing and return the empty string.
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.
All of the text of this specification is normative except sections explicitly marked as non-normative, examples, and notes. [[!RFC2119]]
Examples in this specification are introduced with the words “for example”
or are set apart from the normative text with class="example",
like this:
This is an example of an informative example.
Informative notes begin with the word “Note” and are set apart from the
normative text with class="note", like this:
Note, this is an informative note.
Conformance to CSS Cascading Variables Module is defined for three conformance classes:
A style sheet is conformant to CSS Cascading 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.
A renderer is conformant to CSS Cascading Variables Module if, in addition to interpreting the style sheet as defined by the appropriate specifications, it supports all the features defined by CSS Cascading 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.)
An authoring tool is conformant to CSS Cascading 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.
So that authors can exploit the forward-compatible parsing rules to assign fallback values, CSS renderers must treat as invalid (and ignore as appropriate) any at-rules, properties, property values, keywords, and other syntactic constructs for which they have no usable level of support. In particular, user agents must not 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.
To avoid clashes with future CSS features, the CSS2.1 specification reserves a prefixed syntax for proprietary and experimental extensions to CSS.
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.
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.
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.
Further information on submitting testcases and implementation reports can be found from on the CSS Working Group's website at http://www.w3.org/Style/CSS/Test/. Questions should be directed to the public-css-testsuite@w3.org mailing list.
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:
The specification will remain Candidate Recommendation for at least six months.
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.