CSS @apply Rule

A Collection of Interesting Ideas,

This version:
http://tabatkins.github.io/specs/css-apply-rule
Issue Tracking:
GitHub
Inline In Spec
Editors:
Tab Atkins-Bittner (Google)
(Google)

Abstract

This specification defines the @apply rule, which allows an author to store a set of properties in a named variable, then reference them in other style rules.

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.

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.

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. 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.

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:

This is an example of an informative example.

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

Note, this is an informative note.

Index

Terms defined by this specification

Terms defined by reference

References

Normative References

[CSS-CASCADE-4]
Elika Etemad; Tab Atkins Jr.. CSS Cascading and Inheritance Level 4. 14 January 2016. CR. URL: http://dev.w3.org/csswg/css-cascade/
[CSS-SYNTAX-3]
Tab Atkins Jr.; Simon Sapin. CSS Syntax Module Level 3. 20 February 2014. CR. URL: http://dev.w3.org/csswg/css-syntax/
[CSS-VARIABLES-1]
Tab Atkins Jr.. CSS Custom Properties for Cascading Variables Module Level 1. 3 December 2015. CR. URL: http://dev.w3.org/csswg/css-variables/
[CSSOM-1]
Simon Pieters; Glenn Adams. CSS Object Model (CSSOM). 17 March 2016. WD. URL: https://drafts.csswg.org/cssom/
[HTML]
Ian Hickson. HTML Standard. Living Standard. URL: https://html.spec.whatwg.org/multipage/
[RFC2119]
S. Bradner. Key words for use in RFCs to Indicate Requirement Levels. March 1997. Best Current Practice. URL: https://tools.ietf.org/html/rfc2119
[WebIDL-1]
Cameron McCormack; Boris Zbarsky. WebIDL Level 1. 8 March 2016. CR. URL: https://heycam.github.io/webidl/

Informative References

[CSS-BACKGROUNDS-3]
CSS Backgrounds and Borders Module Level 3 URL: https://drafts.csswg.org/css-backgrounds-3/

IDL Index

interface CSSApplyRule : CSSRule {
  attribute DOMString referencedProperty;
};

Issues Index

Need some way to let you opt into cascading when you want it.
Also, switch CSSStyleRule to inherit from CSSGroupingRule.