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.

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

In this example, we define two custom properties, --item-size and --item-color, that we'll use to define the size (width and height) and background color of the three following items.

<div class="container">
  <div class="item one">Item one</div>
  <div class="item two">Item two</div>
  <div class="item three">Item three</div>
</div>

The following code uses the CSS @property at-rule to define a custom property named --item-size. The property sets the initial value to 40%, limiting valid values to percentage values only. This means, when used as the value for an item's size, its size will always be relative to its parent's size. The property is inheritable.

@property --item-size {
  syntax: "<percentage>";
  inherits: true;
  initial-value: 40%;
}

We define a second custom property, --item-color, using JavaScript instead of CSS. The JavaScript registerProperty() method is equivalent to @property at-rule. The property is defined to have an initial value of aqua, to accept only <color> values, and is not inherited.

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

We use the two custom properties to style the items:

.container {
  display: flex;
  height: 200px;
  border: 1px dashed black;

  /* set custom property values on parent */
  --item-size: 20%;
  --item-color: orange;
}

/* use custom properties to set item size and background color */
.item {
  width: var(--item-size);
  height: var(--item-size);
  background-color: var(--item-color);
}

/* set custom property values on element itself */
.two {
  --item-size: initial;
  --item-color: inherit;
}

.three {
  /* invalid values */
  --item-size: 1000px;
  --item-color: xyz;
}

The two custom properties, --item-size: 20% and --item-color: orange; are set on the container parent, overriding the 40% and aqua default values set when these custom properties were defined. The size is set to be inheritable; the color is not.

For item one, none of these custom properties have been set. The --item-size is inheritable, so the value 20% set on its parent container is used. On the other hand, the property --item-color is not inheritable, so the value orange set on the parent is not considered. Instead the default initial value aqua is used.

For item two, CSS global keywords are set for both custom properties which are valid values for all value types and therefore valid no matter the syntax descriptor value. The --item-size is set to initial and uses the initial-value: 40%; set in the @property declaration. The initial value means theinitialValue value for the property is used. The --item-color is set to inherit, explicitly inheriting the orange value from its parent even though the custom property is set to otherwise not be inherited. This is why item two is orange.

For item three, the --item-size value gets set to 1000px. While 1000px is a <length> value, the @property declaration requires the value be a <percentage>, so the declaration is not valid and is ignored, meaning the inheritable 20% set on the parent is used. The xyz value is also invalid. As registerProperty() set --item-color to not be inherited, the default initial value of aqua is used and not the parent's orange value.

• CSS Properties and Values API
• CSS Painting API
• CSS Typed Object Model
• CSS Houdini