This module introduces the ability to nest one style rule inside another, with the selector of the child rule relative to the selector of the parent rule. This increases the modularity and maintainability of CSS stylesheets.
CSS is a language for describing the rendering of structured documents
(such as HTML and XML)
on screen, on paper, etc.
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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:
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.)
The nesting selector can be desugared
by replacing it with the parent style rule’s selector,
wrapped in a :matches() selector.
For example,
a, b {
& c { color: blue; }
}
is equivalent to
: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.
For example, given the following style rules:
#a, .b {
& c { color: blue; }
}
Then in a DOM structure like
<divid=a><c>foo</c></div>
the & selector has specificity [1,0,0]
because it matches due to the #a selector,
giving the entire color: blue rule a specificity of [1,0,1].
Note: This specificity is intentionally equivalent to that of the desugaring described above.
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.
.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 editing 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 editing more complicated,
and is author-hostile,
so we disallow it as a possibility.
While direct nesting looks nice,
it is somewhat fragile.
Some valid nesting selectors,
like .foo &,
are disallowed,
and editing the selector in certain ways can make the rule invalid unexpectedly.
As well,
some people find the nesting challenging to distinguish visually
from the surrounding declarations.
To aid in all these issues,
this specification defines the @nest rule,
which imposes fewer restrictions on how to validly nest style rules.
Its syntax is:
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;
}
}
/* 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 */
3.3. Mixing Nesting Rules and Declarations
A style rule can have any number of nested style rules inside of it,
of either type,
intermixed with any number of declarations,
in any order.
The relative ordering of nested style rules and other declarations is important;
it’s possible for a given style rule and a nested style rule within it to match the same element,
and if the specificity of the two rules is otherwise equivalent,
the relative order in the stylesheet of the applicable declarations
determines which declaration "wins" the cascade.
4. CSS Object Model Modifications
Add an interface for the @nest rule.
Tie into the general work needed to let rules be nested into style rules.
Conformance
Document conventions
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.
Advisements are normative sections styled to evoke special attention and are
set apart from other normative text with <strong class="advisement">, like
this: UAs MUST provide an accessible alternative.
Conformance classes
Conformance to this specification
is defined for three conformance classes:
A style sheet is conformant to this specification
if all of its statements that use syntax defined in this module are valid
according to the generic CSS grammar and the individual grammars of each
feature defined in this module.
A renderer is conformant to this specification
if, in addition to interpreting the style sheet as defined by the
appropriate specifications, it supports all the features defined
by this specification by parsing them correctly
and rendering the document accordingly. However, the inability of a
UA to correctly render a document due to limitations of the device
does not make the UA non-conformant. (For example, a UA is not
required to render color on a monochrome monitor.)
An authoring tool is conformant to this specification
if it writes style sheets that are syntactically correct according to the
generic CSS grammar and the individual grammars of each feature in
this module, and meet all other conformance requirements of style sheets
as described in this module.
Requirements for Responsible Implementation of CSS
The following sections define several conformance requirements
for implementing CSS responsibly,
in a way that promotes interoperability in the present and future.
Partial Implementations
So that authors can exploit the forward-compatible parsing rules to assign fallback values, CSS renderers must treat as invalid
(and ignore as appropriate)
any at-rules, properties, property values, keywords, and other syntactic constructs
for which they have no usable level of support.
In particular, user agents must not selectively ignore
unsupported property values and honor supported values in a single multi-value property declaration:
if any value is considered invalid (as unsupported values must be),
CSS requires that the entire declaration be ignored.
Implementations of Unstable and Proprietary Features
Once a specification reaches the Candidate Recommendation stage,
implementers should release an unprefixed implementation
of any CR-level feature they can demonstrate
to be correctly implemented according to spec,
and should avoid exposing a prefixed variant of that feature.
To establish and maintain the interoperability of CSS across
implementations, the CSS Working Group requests that non-experimental
CSS renderers submit an implementation report (and, if necessary, the
testcases used for that implementation report) to the W3C before
releasing an unprefixed implementation of any CSS features. Testcases
submitted to W3C are subject to review and correction by the CSS
Working Group.