CSS Nesting Module Level 3

1. Introduction

This section is not normative.

This module describes support for nesting a style rule within another style rule, allowing the inner rule’s selector to reference the elements matched by the outer rule. This feature allows related styles to be aggregated into a single structure within the CSS document, improving readability and maintainability.

1.1. Module Interactions

This module introduces new parser rules that extend the [CSS21] parser model. This module introduces selectors that extend the [SELECTORS4] module.

1.2. Values

This specification does not define any new properties or values.

1.3. Motivation

CSS Rules for even moderately complicated web pages include lots of duplication for the purpose of styling related content. For example, here is a portion of the CSS markup for one version of the [CSS3COLOR] module:

table.colortable td {
  text-align:center;
}
table.colortable td.c {
  text-transform:uppercase;
}
table.colortable td:first-child, table.colortable td:first-child+td {
  border:1px solid black;
}
table.colortable th {
  text-align:center;
  background:black;
  color:white;
}

Nesting allow the grouping of related style rules, like this:

table.colortable {
  & td {
    text-align:center;
    &.c { text-transform:uppercase }
    &:first-child, &:first-child + td { border:1px solid black }
  }
  & th {
    text-align:center;
    background:black;
    color:white;
  }
}

Besides removing duplication, the grouping of related rules improves the readability and maintainability of the resulting CSS.

2. Nesting Selector: the & selector

When using a nested style rule, one must be able to refer to the elements matched by the parent rule; that is, after all, the entire point of nesting. To accomplish that, this specification defines a new selector, the nesting selector, written as an ASCII ampersand &.

When used in the selector of a nested style rule, the nesting selector represents the elements matched by the parent rule. When used in any other context, it represents nothing. (That is, it’s valid, but matches no elements.)

a, b {
  & c { color: blue; }
}

:matches(a, b) c { color: blue; }

The specificity of the nesting selector is equal to the largest specificity among the parent style rule’s selector that match the given element.

#a, .b {
  & c { color: blue; }
}

<div id=a>
  <c>foo</c>
</div>

Note: This specificity is intentionally equivalent to that of the desugaring described above.

The nesting selector is allowed anywhere in a compound selector, even before a type selector, violating the normal restrictions on ordering within a compound selector.

Note: This is required to allow direct nesting. Also, the "type selectors must come first" has no intrinsic reason behind it; it exists because we need to be able to tell simple selectors apart unambiguously when they’re directly appended together in a compound selector, and it’s not clear from .foodiv that it should mean the same as div.foo. An ampersand is unambiguously separable from an ident, tho, so there is no problem with it preceding a type selector, like &div.

3. Nesting Style Rules

Nesting style rules naively inside of other style rules is, unfortunately, problematic—the syntax of a selector is ambiguous with the syntax of a declaration, so an implementation requires unbounded lookahead to tell whether a given bit of text is a declaration or the start of a style rule. As CSS to date requires only a single token of lookahead in its parsing, this drawback is generally considered unacceptable among popular implementations of CSS.

To get around this limitation, this specification defines two methods of nesting style rules inside of other style rules, both designed to be immediately unambiguous with the surrounding declarations. The first, direct nesting, has a somewhat restricted syntax, but imposes minimal additional "weight" in the form of disambiguating syntax, and is suitable for most purposes. The second, the @nest rule, imposes a small syntactic weight to disambiguate it from surrounding declarations, but has no restrictions on the makeup of the selector. The two are otherwise equivalent, and either can be used as desired by the stylesheet author.

3.1. Direct Nesting

A style rule can be directly nested within another style rule if its selector is nest-prefixed.

To be nest-prefixed, a nesting selector must be the first simple selector in the first compound selector of the selector. If the selector is a list of selectors, every complex selector in the list must be nest-prefixed for the selector as a whole to nest-prefixed.

.foo {
  color: blue;
  & > .bar { color: red; }
}
/* equivalent to
   .foo { color: blue; }
   .foo > .bar { color: red; }
 */

.foo {
  color: blue;
  &.bar { color: red; }
}
/* equivalent to
   .foo { color: blue; }
   .foo.bar { color: red; }
 */

.foo, .bar {
  color: blue;
  & + .baz, &.qux { color: red; }
}
/* equivalent to
   .foo, .bar { color: blue; }
   :matches(.foo, .bar) + .baz,
   :matches(.foo, .bar).qux { color: red; }
 */

.foo {
  color: red;
  .bar { color: blue; }
}
/* Invalid because there’s no nesting selector */

.foo {
  color: red;
  .bar & { color:blue; }
}
/* Invalid because & isn’t in the first compound selector */

.foo {
  color: red;
  &.bar, .baz { color: blue; }
}
/* Invalid because the second selector in the list doesn’t
   contain a nesting selector. */

Note: The last invalid example is technically not ambiguous, but it’s still invalid because allowing it would be an editting hazard. Later edits to the stylesheet might remove the first selector in the list, making the other one the new "first selector", and making the rule invalid. Turning an otherwise-innocuous action (like removing a selector from a list) into a possible error makes editting more complicated, and is author-hostile, so we disallow it as a possibility.

3.2. The Nesting At-Rule: @nest

While direct nesting looks nice, it is somewhat fragile. Some valid nesting selectors, like .foo &, are disallowed, and editting the selector in certain ways can make the rule invalid unexpectedly. As well, some people find the nesting difficult to visually distinguish from the surrounding declarations.

To aid in all these issues, this specification defines the @nest rule, which imposes less restrictions on how to validly nest style rules. Its syntax is:

@nest = @nest <selector> { <declaration-list> }

The @nest rule functions identically to a style rule: it starts with a selector, and contains declarations that apply to the elements the selector matches. The only difference is that the selector used in a @nest rule must be nest-containing, which means it contains a nesting selector in it somewhere. A list of selectors is nest-containing if all of its individual complex selectors are nest-containing.

.foo {
  color: red;
  @nest & > .bar {
    color: blue;
  }
}
/* equivalent to
   .foo { color: red; }
   .foo > .bar { color: blue; }
 */

.foo {
  color: red;
  @nest .parent & {
    color: blue;
  }
}
/* equivalent to
   .foo { color: red; }
   .parent .foo { color: blue; }
 */

.foo {
  color: red;
  @nest :not(&) {
    color: blue;
  }
}
/* equivalent to
   .foo { color: red; }
   :not(.foo) { color: blue; }
 */

.foo {
  color: red;
  @nest .bar {
    color: blue;
  }
}
/* Invalid because there’s no nesting selector */

.foo {
  color: red;
  @nest & .bar, .baz {
    color: blue;
  }
}
/* Invalid because not all selectors in the list
   contain a nesting selector */