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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.

The var() CSS function can be used to insert the value of a custom property instead of any part of a value of another property.

var(--header-color, blue);

The var() function cannot be used in 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 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> ]? )

Note: The syntax of the fallback, like that of custom properties, allows commas. For example, var(--foo, red, blue) defines a fallback of red, blue; that is, anything between the first comma and the end of the function is considered a fallback value.


The referenced custom property’s name represented by an 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.
The fallback value, which is used in case the custom property is invalid in the used context. This value may contain any character except some characters with special meaning like newlines, unmatched closing brackets, i.e. ), ], or }, top-level semicolons, or exclamation marks.


: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 */


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]
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. Starting from Gecko 55, the layout.css.variables.enabled preference has been removed completely, so the feature is enabled all the time and can no longer be disabled.

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

[3] See WebKit bug 19660.

See also


 最終更新者: Sebastianz,