This module introduces cascading variables as a new primitive value
type that is accepted by all CSS properties, and custom properties for
defining them.
Status of this document
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 W3C technical reports
index at http://www.w3.org/TR/.
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.
The (archived) public
mailing list www-style@w3.org (see
instructions) 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] …summary of comment…”
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.
1.1. Module
Interactions
This module defines a new type of primitive value, the variable, which is accepted by all
properties.
1.2. Values
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.
2. Defining Custom
Properties: the ‘var-*’
family of properties
This specification defines an open-ended set of properties called custom properties, which are used to define variables.
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.
Custom properties define variables, which can be used
for many purposes. For example, a page that consistently uses a small set
of colors in its design can store the colors in custom properties and use
them with variables:
:root {
main-color: #06c;
accent-color: #006;
}
/* The rest of the CSS file */
#foo h1 {
color: var(main-color);
}
The naming provides a mnemonic for the colors, prevents
difficult-to-spot typos in the color codes, and if the theme colors are
ever changed, focuses the change on one simple spot (the custom property
value) rather than requiring tons of edits across all stylesheets in the
project.
Unlike other CSS properties, custom property names are
case-sensitive. The "var-" prefix must
be written in lower-case.
For example, ‘VAR-FOO’ is
invalid, because the prefix isn't "var-".
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).
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.
Further, the value of a custom property must retain its original
author-given casing, unlike most CSS values which can be safely
lower-cased (because most of CSS is case-insensitive in the ASCII range).
(This requirement does not apply when a custom property's value is
substituted into another property via a variable.)
For example, the following is a valid custom property:
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.
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.
Note: 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:
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:
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’.
3. Using Cascading
Variables: the ‘var()’ notation
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:
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 value.
If the variable named by the first argument is valid, the variable's
value is substituted as normal. If the variable is invalid, and a
<fallback> was provided, the <fallback>
is substituted instead. Otherwise, the result of evaluating the ‘var()’ function will mean that the containing
declaration is invalid at computed-value time.
The fallback value allows for some types of defensive
coding. For example, an author may create a component intended to be
included in a larger application, and use variables to style it so that
it's easy for the author of the larger application to theme the component
to match the rest of the app.
Without fallback, the app author must supply a value for every
variable that your component uses. With fallback, the component author
can supply defaults, so the app author only needs to supply values for
the variables they wish to override.
/* In the component's style: */
.component .header {
color: var(header-color, blue);
}
.component .text {
color: var(text-color, black);
}
/* In the larger application's style: */
.component {
var-text-color: #080;
/* header-color isn't set,
and so remains blue,
the fallback value */
}
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:
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.
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.
Note: 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.
While the CSSStyleDeclaration interface normally contains attributes
that are camel-cased name variants of all CSS properties (and sometimes
also attributes for their canonical names), it must not contain any such
attributes for custom properties. The camel-case trick does not work, as
custom property names are case-sensitive, and there are potentially an
infinity of custom properties, which is incompatible with the normal
behavior of exposing every property whether it was set in the
corresponding declaration block or not.
4.1.1.
Serializing Custom Properties
Custom property names must be serialized with the casing as provided by
the author.
Ordinarily, property names are restricted to the ASCII range
and are case-insensitive, so implementations typically serialize the name
lowercased.
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, invoke
setProperty() by passing varName as the
property argument and varValue as the
value argument.
Note that using setProperty() to set a property
to the empty string instead deletes the property.
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.
For example, given the following style sheet:
div {
var-foo: 16px;
var-Bar: red;
var-foo-bar: 50%;
}
The following lines of script all return something useful:
el = document.querySelector("div");
print(el.style.var.foo); /* Prints the value of "var-foo" */
print(el.style.var.Bar); /* Prints the value of "var-Bar" */
print(el.style.var["foo-bar"]); /* Prints the value of "var-foo-bar" */
On the other hand, the following do not:
print(el.style.varFoo);
/* Custom properties don't exist directly on "style" */
print(el.style.varfoo);
/* Not even if the casing matches. */
print(el.style.var.foo-bar);
/* Retrieves "var-foo" and subtracts a JS variable named "bar",
rather than retrieving the value of "var-foo-bar" */
Iterating over all of the custom properties (for
example, for a JS library to find the ones it knows about and wants to
respond to) is also simple with the var property:
var customProps = el.style.var;
for(customPropName in customProps) {
var customPropValue = customProps[customPropName];
/* More stuff here. */
}
5. Changes from 10 April 2012
Working Draft
The value syntax for custom properties has been nailed down more
precisely.
Case-sensitivity of custom property names has been defined.
The fallback argument was added to the var() function.
A property that is invalid at computed-value time now either goes
inherit or initial, rather than always initial.
CSSVariableComponentValue interface has been dropped, pending the
*ComponentValue interfaces being created at all.
The CSSVariablesDeclaration interface has been added, which stores all
the variables defined by a style rule.
6. Acknowledgments
Many thanks to several people in the CSS Working Group for keeping the
dream of variables alive over the years, particularly Daniel Glazman and
David Hyatt. Thanks to multiple people on the mailing list for helping
contribute to the development of this incarnation of variables,
particularly Brian Kardell, David Baron, François Remy, Roland Steiner,
and Shane Stephens.
7. Conformance
7.1. Document
conventions
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.
7.2.
Conformance classes
Conformance to this specification is defined for three conformance
classes:
A style sheet is conformant to this specification if all of its
statements that use syntax defined in this module are valid according to
the generic CSS grammar and the individual grammars of each feature
defined in this module.
A renderer is conformant to this specification if, in addition to
interpreting the style sheet as defined by the appropriate specifications,
it supports all the features defined by this specification by parsing them
correctly and rendering the document accordingly. However, the inability
of a UA to correctly render a document due to limitations of the device
does not make the UA non-conformant. (For example, a UA is not required to
render color on a monochrome monitor.)
An authoring tool is conformant to this specification if it writes style
sheets that are syntactically correct according to the generic CSS grammar
and the individual grammars of each feature in this module, and meet all
other conformance requirements of style sheets as described in this
module.
7.3. Partial
implementations
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.
7.4.
Experimental implementations
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.
7.5. Non-experimental
implementations
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.