`initial-value` CSS at-rule descriptor

The initial-value descriptor of the @property at-rule specifies the initial value for the registered CSS custom property. It is a required descriptor unless the syntax descriptor value is the universal syntax (*). If required but missing or invalid, the entire @property rule is invalid and ignored.

Syntax

/* Set initial color value */
initial-value: rebeccapurple;

/* Set initial length value */
initial-value: 16px;

Values

A value that matches the type specified in the syntax descriptor. For example, if syntax is <color>, then the initial-value must be a valid color value.

If the value of the syntax descriptor is not the universal syntax definition, the initial-value descriptor has to be a computationally independent value. This means the value can be converted into a computed value without depending on other values, except for "global" definitions independent of CSS. For example, 10px is computationally independent—it doesn't change when converted to a computed value. 2in is also valid, because 1in is always equivalent to 96px. However, 3em is not valid, because the value of an em is dependent on the parent's font-size.

Formal definition

Formal syntax

Examples

Setting an initial value for a custom property

This example shows how to define a custom property --my-color with an initial color value of #c0ffee. This initial value will be used when the property is not inherited (inherits: false) and no other value is set on the element.

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

Using JavaScript CSS.registerProperty():

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

Specifications

Browser compatibility

See also

• Other @property descriptors: inherits and syntax
• Registering CSS custom properties
• CSS Properties and Values API
• CSS Painting API
• CSS Typed Object Model
• Houdini APIs