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Declare and use a CSS custom property
A custom property declaration looks like any other CSS declaration, except its name starts with --. Use var() to substitute its value into a regular property:
:root {
--brand-color: rebeccapurple;
--space-unit: 0.5rem;
}
.button {
background-color: var(--brand-color);
padding: calc(var(--space-unit) * 2);
}Here, :root selects the document’s root element and makes the tokens available to its descendants through inheritance. It is a common place for site-wide values, not a requirement: you can declare a custom property on any element. Names are case-sensitive, so --brand-color and --Brand-color are different properties.
Scope a value and override it for a component
Ordinary double-hyphen custom properties inherit. Their value applies to the element where it is declared and, unless overridden, its descendants. A nearer declaration can change the value used within that subtree:
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.card {
--surface-color: white;
background-color: var(--surface-color);
}
.card--dark {
--surface-color: #222;
}If an element matches both classes, its local --surface-color declaration participates in the normal cascade; descendants ordinarily inherit the resulting value. A declaration on one sibling does not become a value that an unrelated sibling can read. Custom properties follow the cascade and inheritance rather than acting like program-language variables or global text replacements. MDN explains that double-hyphen custom properties are subject to the cascade and inherit from their parent.
Provide a fallback with var()
The optional second argument to var() supplies a value when the referenced custom property is unavailable in the relevant sense, such as an unset, unregistered property:
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.notice {
color: var(--notice-color, #333);
}For a layered fallback, nest another var() call:
.panel {
background-color: var(--panel-color, var(--surface-color, white));
}The browser tries --panel-color, then --surface-color, then white. This runtime fallback does not make a browser that lacks custom-property support understand var(). MDN’s var() reference documents the function and its fallback behavior.
Know when a substituted value makes a declaration invalid
A custom property stores a token value; ordinary custom properties do not validate that value against the property where it will eventually be used. The substituted result still has to be valid for the consuming property:
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:root {
--text-color: 16px;
}
p {
color: var(--text-color);
}16px is a length, not a valid color, so the resulting color declaration is invalid at computed-value time. A var() fallback does not rescue an invalid value for the consuming property: the custom property exists, so its value is substituted, and the resulting declaration fails. Check that a token’s value is suitable for every property that uses it.
Register a custom property with @property
For advanced cases, @property lets you declare a custom property’s syntax, inheritance behavior, and initial value. This can constrain values and enable typed animation:
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@property --progress {
syntax: "<percentage>";
inherits: false;
initial-value: 0%;
}
.progress-bar {
width: var(--progress);
}Unlike an ordinary custom property, this registered property is configured not to inherit, has a declared percentage syntax, and has an initial value. That initial value also matters for fallback behavior: a registered property with a non-universal syntax and an initial value may resolve to that value instead of being unavailable for the var() fallback. MDN marks @property Baseline 2024; check compatibility for the browsers and embedded webviews you need to support.
Choose ordinary or registered properties
| Behavior | Ordinary custom property | Registered with @property |
|---|---|---|
| Value syntax | Not constrained by a declared syntax | Can declare a syntax, such as <percentage> |
| Inheritance | Inherits by default | Set explicitly with inherits |
| Initial value | No registration-defined initial value | Can define one with initial-value |
| Typed animation | Not available as a registered typed value | Registered typed values can be animated |
| Availability guidance | var() is widely available; MDN says it has been available across browsers since April 2017 |
MDN marks @property Baseline 2024 |
MDN’s availability labels are documentation guidance, not a guarantee for every browser version or embedded webview. See its var() compatibility information and @property compatibility information for your target environment.
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Know where var() cannot be used
var() substitutes a value inside a property declaration. It cannot parameterize CSS syntax outside property values, including:
- Selectors or property names
- Media-query conditions
- Container-query conditions
Write responsive query conditions directly, then use custom properties in declarations inside the matching rules:
:root {
--page-gutter: 1rem;
}
@media (min-width: 48rem) {
.layout {
padding-inline: var(--page-gutter);
}
}Troubleshoot common custom-property problems
- The value appears unset: Check the spelling and capitalization of the
--name, and confirm the declaration applies to the element or one of its ancestors. - A descendant gets an unexpected value: Inspect the cascade for a closer declaration or another rule that overrides the inherited token.
- The fallback does not appear: If the custom property is set but its value is invalid for the consuming property, the fallback is not used. Make the token valid for that property or provide a suitable value at the declaration site.
- A query condition does not respond to a token: Custom properties cannot supply media- or container-query conditions. Put the breakpoint directly in the query.
- Older browsers fail to understand the CSS: A
var()fallback is not a polyfill for browsers without custom-property support. Check the target browsers’ compatibility before relying on the feature.
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