CSS @apply Rule

1. Introduction

CSS custom properties enable authors to define reusable values, give them names, then invoke them throughout the stylesheet. This makes it easy to keep a page’s theme consistent when changes are made, because the theming values are defined in a central place.

But custom properties can hold more than just values—they can also be used to hold sets of declarations. The @apply rule takes these sets of declarations and inlines them in another style rule, serving a purpose analogous to what the var() function does for individual values.

2. Defining Custom Sets of Properties

To define a custom property set for use with @apply, you simply use a custom property with a value of a {}-wrapped block of properties.

:root {
  --toolbar-theme: {
    background-color: hsl(120, 70%, 95%);
    border-radius: 4px;
    border: 1px solid var(--theme-color late);
  };
  --toolbar-title-theme: {
    color: green;
  };
}

.toolbar {
  @apply --toolbar-theme;
}
.toolbar > .title {
  @apply --toolbar-title-theme;
}

.warning {
  --toolbar-title-theme: {
    color: red;
    font-weight: bold;
  };
}

.foo {
  color: red;
  background: white;
}
#bar {
  color: blue;
}

.foo {
  --my-theme: {
    color: red;
    background: white;
  };
}
#bar {
  --my-theme: {
    color: blue;
  };
}

Need some way to let you opt into cascading when you want it.

2.1. Changes to Custom Property Processing

If the value of a custom property contains an @apply rule, the @apply rule must be valid according to the specified @apply grammar. If not, the custom property is invalid and must be ignored.

@apply rules in custom property values are substituted at computed value time, identically to var() functions.

For this purpose, an @apply rule is composed of a sequence of tokens starting with the @apply token, and ending at (and including) the first same-level semicolon, the end of the simple block the @apply token is in, or the end of the custom property’s value.

The computed value of a custom property

3. Using Custom Sets of Properties: the @apply rule

Once a custom property set has been declared, the @apply rule inlines it into a style rule. It’s syntax is:

@apply = @apply <custom-property-name> ;

The @apply rule is only valid inside of a style rule. Using it outside of a style rule, or inside any other rule, is invalid and causes the @apply to be ignored.

.foo {
  color: blue;
  @apply --foo-styles;
}

.foo {
  color: blue;
}

@apply --top-level-is-invalid;

@keyframes foo {
  from { color: red; }
  to { color: blue; }
  @apply --this-is-not-a-style-rule;
}

For the purposes of the cascade, the @apply rule must be treated as if it were replaced by the properties in the custom property set that is the value of the custom property it references.

Note: Within the CSSOM, the @apply rule is not replaced; examining the style rule will show it as having the @apply rule in its .childRules attribute, and the properties in the custom property set will not be visible in any way.

If the custom property that the @apply rule references does not define a valid custom property set, the @apply rule is treated, for the purposes of the cascade, as if it were replaced with nothing. It is not invalid, however. (For example, it is not dropped from the CSSOM.)

4. Processing @apply Rules

To process @apply rules:

1. Inherit as normal.

2. Do var() substitution into custom properties only.

3. Do @apply substitution.

4. Re-do var() substitutions based on new property info.

5. CSSOM

interface CSSApplyRule : CSSRule {
  attribute DOMString referencedProperty;
};

referencedProperty, of type DOMString

The custom property that the @apply rule is referencing.

Upon setting, if the value is not a valid <custom-property-name>, ignore the set and throw a SyntaxError.

Also, switch CSSStyleRule to inherit from CSSGroupingRule.