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.

There are many ways to apply sets of declarations to an element. In particular, the common way is to just create a style rule, and apply it to the desired elements via a selector. However, this requires the elements you wish to target to already have the right features for a selector, or else you have to alter your markup, or write a complex selector that targets them precisely with their varied features (and which needs to be maintained as you alter the page markup and change the selectors targetting those elements). This also requires careful management of specificity, as the rule has to interact with the existing rules styling the elements.

The @apply rule allows this reuse to be inlined into the existing selectors you’re already using, reducing the amount of effort required to keep your stylesheet consistent as things change in the page. It also avoids the need to manage specificity any more than you already do, as the properties are inlined alongside the existing properties, in your existing style rules.

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.

For example, one might define a toolbar theme as a custom property set on the root element of the document, and use it on your toolbars:

: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;
}

Then, we can override the theme for toolbars inside of "warning" elements:

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

We don’t have to worry about the internal structure of the toolbars, or precisely what internal elements use the styles. Simply overriding the custom property will automatically do the right thing.

Note that custom property sets override each other wholly, rather than cascading together like colliding style rules do.

For example, if these two style rules applied to the same element:

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

The #bar rule will win due to having a higher specificity, so its color:blue rule will apply to the element, but the background:white rule from the .foo rule also applies, since the #bar rule did not override background.

However, if these were instead defined as custom property sets:

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

Then when an element uses the --my-theme custom property set, it will receive only the color:blue declaration. The background:white declaration from the .foo rule is ignored completely, as its rule lost the specificity battle.

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

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.

Here’s a valid example of @apply usage:

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

Here’s several invalid example of @apply usage:

.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. 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.

Conformance

Conformance requirements are expressed with a combination of descriptive assertions and RFC 2119 terminology. The key words “MUST”, “MUST NOT”, “REQUIRED”, “SHALL”, “SHALL NOT”, “SHOULD”, “SHOULD NOT”, “RECOMMENDED”, “MAY”, and “OPTIONAL” in the normative parts of this document are to be interpreted as described in RFC 2119. However, for readability, these words do not appear in all uppercase letters in this specification.

All of the text of this specification is normative except sections explicitly marked as non-normative, examples, and notes. [RFC2119]

Examples in this specification are introduced with the words “for example” or are set apart from the normative text with class="example", like this:

Informative notes begin with the word “Note” and are set apart from the normative text with class="note", like this:

Jan	FEB	Apr
	22
2015	2016	2017

CSS @apply Rule

A Collection of Interesting Ideas, 25 August 2015

Abstract

Table of Contents

1. Introduction

2. Defining Custom Sets of Properties

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

4. CSSOM

Conformance

Index

Terms defined by this specification

Terms defined by reference

References

Normative References

IDL Index

Issues Index