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var()

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

Try it

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.)

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()> = 
  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.

Values

<custom-property-name>

A 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.

<declaration-value>

The custom property's 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. The fallback value can itself be a custom property using the var() syntax.

Note: var(--a,) is valid, specifying that if the --a custom property is invalid or missing, the var() should be replaced with nothing.

Examples

Using a custom property set on :root

CSS

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

body {
  background-color: var(--main-bg-color);
}

Result

Here, the value of the background-color property has been set via the custom property --main-bg-color. So the background color of the HTML body will be pink.

Using a custom property before it is set

CSS

body {
  background-color: var(--main-bg-color);
}

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

Result

In this example, the background color of the HTML body will be pink even though the custom property is set later.

Using a custom property set in another file

HTML

<!DOCTYPE html>
<html lang="en-US">
  <head>
    <meta charset="utf-8" />
    <link rel="stylesheet" href="1.css"></link>
    <link rel="stylesheet" href="2.css"></link>
  </head>
  <body>
  </body>
</html>

CSS

/* 1.css */
body {
  background-color: var(--main-bg-color);
}

/* 2.css */
:root {
  --main-bg-color: pink;
}

Result

The background color of the HTML body will be pink in this case even though the custom property is declared in another file.

Custom properties with fallbacks for use when the property has not been set

HTML

<div class="component">
  <h1 class="header">Header</h1>
  <p class="text">Text</p>
</div>

CSS

/* In the component's style: */
.component .header {
  /* header-color isn't set, and so remains blue, the fallback value */
  color: var(--header-color, blue);
}

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

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

Result

Since --header-color isn't set, the text "Header" will be blue, the fallback value.

Using a custom property as a fallback

CSS

:root {
  --backup-bg-color: teal;
}

body {
  background-color: var(--main-bg-color, var(--backup-bg-color, white));
}

Result

Since --main-bg-color isn't set, the body's background-color will fall back to --backup-bg-color, which is teal.

Specifications

Specification
CSS Custom Properties for Cascading Variables Module Level 1
# using-variables

Browser compatibility

Desktop Mobile
Chrome Edge Firefox Internet Explorer Opera Safari WebView Android Chrome Android Firefox for Android Opera Android Safari on IOS Samsung Internet
var() 49 15 31 No 36 9.1 50 49 31 36 9.3 5.0

See also

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Licensed under the Creative Commons Attribution-ShareAlike License v2.5 or later.
https://developer.mozilla.org/en-US/docs/Web/CSS/var()