Custom properties (--*): CSS variables

Property names that are prefixed with --, like --example-name, represent custom properties that contain a value that can be used in other declarations using the var() function.

Custom properties are scoped to the element(s) they are declared on, and participate in the cascade: the value of such a custom property is that from the declaration decided by the cascading algorithm.

Syntax

--some-keyword: left;
--some-color: #123456;
--some-complex-value: 3px 6px rgb(20 32 54);

<declaration-value>

This value matches any sequence of one or more tokens, so long as the sequence does not contain any disallowed token. It represents the entirety of what a valid declaration can have as its value.

Example

Basic example

HTML

<p id="firstParagraph">
  This paragraph should have a blue background and yellow text.
</p>
<p id="secondParagraph">
  This paragraph should have a yellow background and blue text.
</p>
<div id="container">
  <p id="thirdParagraph">
    This paragraph should have a green background and yellow text.
  </p>
</div>

CSS

:root {
  --first-color: #1166ff;
  --second-color: #ffff77;
}

#firstParagraph {
  background-color: var(--first-color);
  color: var(--second-color);
}

#secondParagraph {
  background-color: var(--second-color);
  color: var(--first-color);
}

#container {
  --first-color: #229900;
}

#thirdParagraph {
  background-color: var(--first-color);
  color: var(--second-color);
}

Result

Registering custom properties with @property

In this example, we use the @property at-rule to register a custom property.

HTML

Our HTML includes an ordered list (<ol>) containing three list items (<li>).

<ol>
  <li class="one">Item one</li>
  <li class="two">Item two</li>
  <li class="three">Item three</li>
</ol>

CSS

We use the @property at-rule to register two custom properties.

@property --itemSize {
  syntax: "<length> | <percentage>";
  inherits: true;
  initial-value: 200px;
}

@property --borderWidth {
  syntax: "<length>";
  inherits: false;
  initial-value: 10px;
}

We try to override the custom property values. The values set on .two are valid while the values set on .three are invalid.

ol {
  --itemSize: 100px;
  --borderWidth: 1px;
}
.two {
  --itemSize: initial;
  --borderWidth: inherit;
}
.three {
  --itemSize: large;
  --borderWidth: 3%;
}

We use the two custom properties to style the items, setting the border and width for all the items at once:

li {
  width: var(--itemSize);
  border: var(--borderWidth) solid red;
  background-color: yellow;
  margin-bottom: 10px;
}

Results

The --itemSize property is inheritable; the --borderWidth is not. The properties are set on the ol parent, overriding the default values defined in their registration. Item one inherits the size but not the border width from the OL. The global keywords, declared for .two, are valid for <length>, so are used. The values in .three are invalid ("large" is not a <length-percentage> and 3% is not a <length>). See

Specifications

Browser compatibility

See also

• The var() function
• @property at-rule
• Using CSS custom properties (variables) guide
• Registering CSS custom properties guide
• CSS custom properties for cascading variables module