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This is an experimental technology
Because this technology's specification has not stabilized, check the compatibility table for usage in various browsers. Also note that the syntax and behavior of an experimental technology is subject to change in future versions of browsers as the specification changes.

Summary

The var() function can be used instead of any part of a value in any property on an element. The var() function 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.)

Syntax

The first argument to the function is the name of the custom property to be substituted. An optional second argument to the function serves as a fallback value. If the custom property referenced by the first argument is invalid, the function uses the second value.

var( <custom-property-name> [, <declaration-value> ]? )

Values

<custom-property-name>
The production corresponds to this: It’s defined as any valid identifier that starts with two dashes. Custom properties are solely for use by authors and users; CSS will never give them a meaning beyond what is presented here.
<declaration-value>
The <declaration-value> production matches any sequence of one or more tokens.  So, as long as the sequence does not contain <bad-string-token>, <bad-url-token>, unmatched <)-token>, <]-token>, or <}-token>, or top-level <semicolon-token> tokens or <delim-token> tokens with a value of "!" ,it represents the entirety of what a valid declaration can have as its value.

Examples

:root {
  --main-bg-color: pink;
}

body {
  background-color: var(--main-bg-color);
}
/* Fallback */
/* 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 {
  --text-color: #080; /* header-color isn’t set, and so remains blue, the fallback value */
}

Specifications

Specification Status Comment
CSS Custom Properties for Cascading Variables Module Level 1
The definition of 'var()' in that specification.
Working Draft Initial definition

Browser compatibility

Feature Chrome Firefox (Gecko) Internet Explorer Opera Safari (WebKit)
Basic support 48[2]
49
29 (29)[1]
31 (31)
No support 36 9.3[3]
Feature Android Firefox Mobile (Gecko) IE Phone Opera Mobile Safari Mobile
Basic support 50 29 (29) No support 37 9.3[3]

[1] This feature is implemented behind the preference layout.css.variables.enabled, defaulting to false and using the old var-variablename syntax in Gecko 29. Starting from Gecko 31 the preference is enabled by default and the new --variablename syntax is used.

[2] Chrome 48 enabled through the "Experimental Web Platform features" flag in chrome://flags.

[3] See WebKit bug 19660.

See also

Document Tags and Contributors

 Contributors to this page: Sebastianz, DevelX, PushpitaPikuDey, rolfedh, andreww2012, yisibl
 Last updated by: Sebastianz,