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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); /* header-color isn’t set, and so remains blue, the fallback value */

.component .text {
  color: var(--text-color, black);

/* In the larger application’s style: */
.component {
  --text-color: #080;


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


FeatureChromeEdgeFirefoxInternet ExplorerOperaSafari
Basic support





29 — 552 3

No36 Yes
FeatureAndroid webviewChrome for AndroidEdge mobileFirefox for AndroidIE mobileOpera AndroidiOS Safari
Basic support50 ?15


29 — 552 3

No37 Yes

1. From version 48: this feature is behind the Enable experimental Web Platform features preference. To change preferences in Chrome, visit chrome://flags.

2. From Firefox 29 until Firefox 31, this feature was implemented by the var-variablename syntax.

3. From version 29 until version 55 (exclusive): this feature is behind the layout.css.variables.enabled preference (needs to be set to true). To change preferences in Firefox, visit about:config.

See also


 最終更新者: fscholz,