@property

The @property CSS at-rule is part of the CSS Houdini umbrella of APIs, it allows developers to explicitly define their CSS custom properties, allowing for property type checking, setting default values, and define whether a property can inherit values or not.

The @property rule represents a custom property registration directly in a stylesheet without having to run any JS. Valid @property rules result in a registered custom property, as if CSS.registerProperty had been called with equivalent parameters.

Syntax

css

@property --property-name {
  syntax: "<color>";
  inherits: false;
  initial-value: #c0ffee;
}

Descriptors

syntax

Describes the allowable syntax for the property.

inherits

Controls whether the custom property registration specified by @property inherits by default.

initial-value

Sets the initial value for the property.

A valid @property rule represents a custom property registration, with the property name being the serialization of the in the rule's prelude.

@property rules require a syntax and inherits descriptor; if either are missing, the entire rule is invalid and must be ignored. The initial-value descriptor is optional only if the syntax is the universal syntax definition, otherwise the descriptor is required; if it's missing, the entire rule is invalid and must be ignored.

Unknown descriptors are invalid and ignored, but do not invalidate the @property rule.

Examples

Add type checking to --my-color custom property, as a color, a default value, and not allow it to inherit its value:

Using CSS @property at-rule:

css

@property --my-color {
  syntax: "<color>";
  inherits: false;
  initial-value: #c0ffee;
}

Using JavaScript CSS.registerProperty:

js

window.CSS.registerProperty({
  name: "--my-color",
  syntax: "<color>",
  inherits: false,
  initialValue: "#c0ffee",
});

Formal syntax

@property = 
  @property <custom-property-name> { <declaration-list> }

Specifications

Specification
CSS Properties and Values API Level 1
# at-property-rule

Browser compatibility

BCD tables only load in the browser

See also