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<pre class='metadata'>
Title: CSS Nesting Module Level 1
Shortname: css-nesting
Level: 1
Status: ED
Work Status: Exploring
Group: CSSWG
ED: https://drafts.csswg.org/css-nesting/
TR: https://www.w3.org/TR/css-nesting-1/
Editor: Tab Atkins-Bittner, Google, http://xanthir.com/contact/, w3cid 42199
Former Editor: Adam Argyle, Google, https://nerdy.dev, w3cid 112669
Abstract: 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.
Ignored Terms: cssText
Default Highlight: css
Include MDN Panels: no
WPT Path Prefix: css/css-nesting/
WPT Display: open
</pre>
<pre class=link-defaults>
spec:css-color-4; type:property; text:color
spec:css-values-4; type:dfn; text:identifier
spec:cssom-1; type:dfn;
text:child css rules
text:specified order
spec:selectors-4; type:dfn;
text: combinators
text: simple selector
text: type selector
</pre>
<h2 id="intro">
Introduction</h2>
<em>This section is not normative.</em>
This module describes support for
7803
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.
<wpt title="General tests for nesting">
block-skipping.html
delete-other-rule-crash.html
double-parent-pseudo-in-placeholder-crash.html
has-nesting.html
invalidation-001.html
invalidation-002.html
invalidation-003.html
invalidation-004.html
nested-error-recovery.html
nesting-basic.html
nesting-revert-rule.tentative.html
parent-pseudo-in-placeholder-crash.html
pseudo-part-crash.html
pseudo-where-crash.html
supports-is-consistent.html
</wpt>
<h3 id="placement">
Module Interactions</h3>
This module introduces new parser rules that extend the [[!CSS21]] parser model.
It introduces selectors that extend the [[SELECTORS-4]] module.
It extends and modifies some IDL and algorithms defined in the [[CSSOM-1]] module.
<h3 id="values">
Values</h3>
This specification does not define any new properties or values.
<h2 id="explainer">
Explainer</h2>
<em>This section is non-normative.</em>
Imagine you have some CSS that you’d like to write in a more compact way.
<pre>
.foo {
color: green;
}
.foo .bar {
font-size: 1.4rem;
}
</pre>
With Nesting, you can write such code as:
<pre>
.foo {
color: green;
.bar {
font-size: 1.4rem;
}
}
</pre>
If you’ve been nesting styles in Sass
or other CSS preprocessors,
you will find this very familiar.
You can nest any rules inside of a parent style rule:
<pre>
main {
div { ... }
.bar { ... }
#baz { ...}
:has(p) { ... }
::backdrop { ... }
[lang|="zh"] { ... }
* { ... }
}
</pre>
By default, the child rule's selector
is assumed to connect to the parent rule
by a [=descendant combinator=],
but you can start the nested selector
with any combinator to change that:
<pre>
main {
+ article { ... }
> p { ... }
~ main { ... }
}
</pre>
The new ''&'' selector lets you refer to
the elements matched by the parent selector explictly,
so the previous examples could have been written as:
<pre>
main {
& + article { ... }
& > p { ... }
& ~ main { ... }
}
</pre>
But you can place the ''&'' in other locations
within the nested selector,
to indicate other types of relationships
between the parent and child rule.
For example,
this CSS:
<pre>
ul {
padding-left: 1em;
}
.component ul {
padding-left: 0;
}
</pre>
Can be rewritten using Nesting as:
<pre>
ul {
padding-left: 1em;
.component & {
padding-left: 0;
}
}
</pre>
Again, the ''&'' gives you a way to say
“this is where I want the nested selector to go”.
It’s also handy when you don’t want a space between your selectors.
For example:
<pre>
a {
color: blue;
&:hover {
color: lightblue;
}
}
</pre>
Such code yields the same result as <code>a:hover {</code>.
Without the ''&'',
you’d get <code>a :hover {</code>--
notice the space between <code>a</code> and <code>:hover</code>--
which would fail to style your hover link.
You can nest more than one layer deep--
nesting CSS inside already-nested CSS--
in as many levels as you desire.
You can mix Nesting with
Container Queries, Supports Queries, Media Queries, and/or Cascade Layers
however you want.
(Nearly) anything can go inside of anything.
Nesting Style Rules {#nesting}
==============================
Style rules can be nested inside of other styles rules.
These <dfn export lt="nested style rule|nesting style rule">nested style rules</dfn>
act exactly like ordinary style rules--
associating properties with elements via selectors--
but they "inherit" their parent rule's selector context,
allowing them to further build on the parent's selector
without having to repeat it,
possibly multiple times.
A [=nested style rule=] is exactly like a normal style rule,
except that it can use [=relative selectors=],
which are implicitly relative
to the elements matched by the parent rule.
<wpt>
implicit-nesting.html
implicit-nesting-ident.html
implicit-nesting-ident-recovery.html
implicit-parent-insertion-crash.html
</wpt>
<div class=example>
That is,
a nested style rule like:
<pre class=lang-css>
.foo {
color: red;
a {
color: blue;
}
}
</pre>
is valid,
and equivalent to:
<pre class=lang-css>
.foo {
color: red;
}
.foo a {
color: blue;
}
</pre>
The nested rule can also use the [=nesting selector=]
to directly refer to the parent rule's matched elements,
or use [=relative selector=] syntax
to specify relationships other than "descendant".
<pre class=lang-css>
.foo {
color: red;
&:hover {
color: blue;
}
}
/* equivalent to: */
.foo { color: red; }
.foo:hover { color: blue; }
</pre>
<pre class=lang-css>
.foo {
color: red;
+ .bar {
color: blue;
}
}
/* equivalent to: */
.foo { color: red; }
.foo + .bar { color: blue; }
</pre>
</div>
<wpt>
nested-rule-cssom-invalidation.html
</wpt>
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Syntax {#syntax}
------------------------
The contents of [=style rules=]
now accepts [=nested style rules=]
and [=at-rules=],
in addition to the existing [=declarations=].
[=Nested style rules=] differ from non-nested rules
in the following ways:
* A [=nested style rule=] accepts a <<relative-selector-list>>
as its prelude
(rather than just a <<selector-list>>).
Any [=relative selectors=]
are relative to the elements represented by the [=nesting selector=].
* If a selector in the <<relative-selector-list>>
does not start with a [=combinator=]
but does [=contain the nesting selector=],
it is interpreted as a non-[=relative selector=].
The precise details of how nested style rules are parsed
are defined in [[!CSS-SYNTAX-3]].
An invalid [=nested style rule=] is ignored,
along with its contents,
but does not invalidate its parent rule.
Nested rules with [=relative selectors=]
include the specificity of their implied [=nesting selector=].
For example, ''.foo { > .bar {...}}'' and ''.foo { & > .bar {...}}''
have the same specificity for their inner rule.
<div class=note>
Some CSS-generating tools that preprocess nesting
will concatenate selectors as strings,
allowing authors to build up a <em>single</em> [=simple selector=]
across nesting levels.
This is sometimes used with hierarchical name patterns
like <a href="https://getbem.com/introduction/">BEM</a>
to reduce repetition across a file,
when the selectors themselves have significant repetition internally.
For example, if one component uses the class ''.foo'',
and a nested component uses ''.fooBar'',
you could write this in <a href="https://sass-lang.com/">Sass</a> as:
<xmp class="lang-css">
.foo {
color: blue;
&Bar { color: red; }
}
/* In Sass, this is equivalent to
.foo { color: blue; }
.fooBar { color: red; }
*/
</xmp>
This is not allowed in CSS,
as nesting is not a syntax transformation,
but rather matches on the actual elements the parent selector matches.
It is also true that the selector ''&Bar'' is invalid in CSS in the first place,
as the ''Bar'' part is a type selector,
which must come first in the compound selector.
(That is, it must be written as ''Bar&''.)
So, luckily, there is no overlap between CSS Nesting
and the preprocessor syntax.
</div>
<div algorithm>
A selector is said to <dfn export>contain the nesting selector</dfn>
if, when it was [=CSS/parsed=] as any type of selector,
a <<delim-token>> with the value "&" (U+0026 AMPERSAND) was encountered.
Note: This is phrased in this explicit manner
so as to catch cases like '':is(:unknown(&), .bar)'',
where an unknown selector
(which, being unknown, we have no way of knowing
whether the argument is <em>meant</em> to be parsed as a selector or not)
is the only part of the selector that contains an ''&''.
As that <em>might</em> be a perfectly valid selector
that's only supported by newer browsers,
and we don't want parsing to be dependent on unrelated versioning issues,
we treat it as still [=containing the nesting selector=].
</div>
If a <<forgiving-selector-list>> has an item that [=contains the nesting selector=]
but is invalid,
that item is preserved exactly as-is
rather than being discarded.
(This does not change the matching behavior of the selector--
an invalid selector still fails to match anything--
just the serialization of the selector.)
<wpt>
nest-containing-forgiving.html
parsing.html
</wpt>
Issue: The preceding paragraph needs to move to Selectors
when we move ''&'' itself to Selectors;
I'm monkey-patching for convenience here.
<!-- Big Text: example
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<h3 id=syntax-examples>
Examples</h3>
<pre class=lang-css>
<b>/* & can be used on its own */</b>
.foo {
color: blue;
& > .bar { color: red; }
> .baz { color: green; }
}
/* equivalent to
.foo { color: blue; }
.foo > .bar { color: red; }
.foo > .baz { color: green; }
*/
<b>/* or in a compound selector,
refining the parent's selector */</b>
.foo {
color: blue;
&.bar { color: red; }
}
/* equivalent to
.foo { color: blue; }
.foo.bar { color: red; }
*/
<b>/* multiple selectors in the list are all
relative to the parent */</b>
.foo, .bar {
color: blue;
+ .baz, &.qux { color: red; }
}
/* equivalent to
.foo, .bar { color: blue; }
:is(.foo, .bar) + .baz,
:is(.foo, .bar).qux { color: red; }
*/
<b>/* & can be used multiple times in a single selector */</b>
.foo {
color: blue;
& .bar & .baz & .qux { color: red; }
}
/* equivalent to
.foo { color: blue; }
.foo .bar .foo .baz .foo .qux { color: red; }
*/
<b>/* & doesn't have to be at the beginning of the selector */</b>
.foo {
color: red;
.parent & {
color: blue;
}
}
/* equivalent to
.foo { color: red; }
.parent .foo { color: blue; }
*/
.foo {
color: red;
:not(&) {
color: blue;
}
}
/* equivalent to
.foo { color: red; }
:not(.foo) { color: blue; }
*/
<b>/* But if you use a <l>[=relative selector=]</l>,
an initial & is implied automatically */</b>
.foo {
color: red;
+ .bar + & { color: blue; }
}
/* equivalent to
.foo { color: red; }
.foo + .bar + .foo { color: blue; }
*/
<b>/* Somewhat silly, but & can be used all on its own, as well. */</b>
.foo {
color: blue;
& { padding: 2ch; }
}
/* equivalent to
.foo { color: blue; }
.foo { padding: 2ch; }
// or
.foo {
color: blue;
padding: 2ch;
}
*/
<b>/* Again, silly, but can even be doubled up. */</b>
.foo {
color: blue;
&& { padding: 2ch; }
}
/* equivalent to
.foo { color: blue; }
.foo.foo { padding: 2ch; }
*/
<b>/* The parent selector can be arbitrarily complicated */</b>
.error, #404 {
&:hover > .baz { color: red; }
}
/* equivalent to
:is(.error, #404):hover > .baz { color: red; }
*/
.ancestor .el {
.other-ancestor & { color: red; }
}
/* equivalent to
.other-ancestor :is(.ancestor .el) { color: red; }
<b>/* As can the nested selector */</b>
.foo {
& :is(.bar, &.baz) { color: red; }
}
/* equivalent to
.foo :is(.bar, .foo.baz) { color: red; }
*/
<b>/* Multiple levels of nesting "stack up" the selectors */</b>
figure {
margin: 0;
> figcaption {
background: hsl(0 0% 0% / 50%);
> p {
font-size: .9rem;
}
}
}
/* equivalent to
figure { margin: 0; }
figure > figcaption { background: hsl(0 0% 0% / 50%); }
figure > figcaption > p { font-size: .9rem; }
*/
<b>/* Example usage with Cascade Layers */</b>
@layer base {
html {
block-size: 100%;
body {
min-block-size: 100%;
}
}
}
/* equivalent to
@layer base {
html { block-size: 100%; }
html body { min-block-size: 100%; }
}
*/
<b>/* Example nesting Cascade Layers */</b>
@layer base {
html {
block-size: 100%;
@layer support {
body {
min-block-size: 100%;
}
}
}
}
/* equivalent to
@layer base {
html { block-size: 100%; }
}
@layer base.support {
html body { min-block-size: 100%; }
}
*/
<b>/* Example usage with Scoping */</b>
@scope (.card) to (> header) {
:scope {
inline-size: 40ch;
aspect-ratio: 3/4;
> header {
border-block-end: 1px solid white;
}
}
}
/* equivalent to
@scope (.card) to (> header) {
:scope { inline-size: 40ch; aspect-ratio: 3/4; }
:scope > header { border-block-end: 1px solid white; }
}
*/
<b>/* Example nesting Scoping */</b>
.card {
inline-size: 40ch;
aspect-ratio: 3/4;
@scope (&) to (> header > *) {
:scope > header {
border-block-end: 1px solid white;
}
}
}
/* equivalent to
.card { inline-size: 40ch; aspect-ratio: 3/4; }
@scope (.card) to (> header > *) {
:scope > header { border-block-end: 1px solid white; }
}
*/
</pre>
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Nesting Other At-Rules {#conditionals}
--------------------------------------
In addition to [=nested style rules=],
this specification allows <dfn export>nested group rules</dfn>
inside of [=style rules=]:
any at-rule whose body contains [=style rules=]
can be nested inside of a [=style rule=] as well,
unless otherwise specified.
When nested in this way,
the contents of a [=nested group rule=]'s block
are parsed as <<block-contents>>
rather than <<rule-list>>:
* [=Style rules=] are [=nested style rules=],
with their [=nesting selector=] taking its definition
from the nearest ancestor [=style rule=].
* Properties can be directly used,
acting as if they were nested in a [=nested declarations rule=].
<div class=note>
Specifically, these rules are capable of being [=nested group rules=]:
* all the [=conditional group rules=] (''@container'', ''@media'', ''@supports'')
* ''@layer''
* ''@scope''
</div>
<wpt>
conditional-properties.html
conditional-rules.html
invalid-inner-rules.html
supports-rule.html
</wpt>
The meanings and behavior of such [=nested group rules=]
is otherwise unchanged,
unless otherwise specified.
<div class="example">
For example, the following conditional nestings are valid:
<pre class="lang-css">
<b>/* Properties can be directly used */</b>
.foo {
display: grid;
@media (orientation: landscape) {
grid-auto-flow: column;
}
}
/* equivalent to: */
.foo {
display: grid;
}
@media (orientation: landscape) {
.foo {
grid-auto-flow: column
}
}
/* and also equivalent to the unnested: */
.foo { display: grid; }
@media (orientation: landscape) {
.foo {
grid-auto-flow: column;
}
}
<b>/* Conditionals can be further nested */</b>
.foo {
display: grid;
@media (orientation: landscape) {
grid-auto-flow: column;
@media (min-width > 1024px) {
max-inline-size: 1024px;
}
}
}
/* equivalent to */
.foo { display: grid; }
@media (orientation: landscape) {
.foo {
grid-auto-flow: column;
}
}
@media (orientation: landscape) and (min-width > 1024px) {
.foo {
max-inline-size: 1024px;
}
}
<b>/* Example nesting Cascade Layers */</b>
html {
@layer base {
block-size: 100%;
@layer support {
& body {
min-block-size: 100%;
}
}
}
}
/* equivalent to */
@layer base {
html { block-size: 100%; }
}
@layer base.support {
html body { min-block-size: 100%; }
}
<b>/* Example nesting Scoping */</b>
.card {
inline-size: 40ch;
aspect-ratio: 3/4;
@scope (&) {
:scope {
border: 1px solid white;
}
}
}
/* equivalent to */
.card { inline-size: 40ch; aspect-ratio: 3/4; }
@scope (.card) {
:scope { border-block-end: 1px solid white; }
}
</pre>
</div>
Runs of consecutive directly-nested properties
are automatically wrapped in [=nested declarations rules=].
(This is observable in the CSSOM.)
<wpt>
nesting-layer.html
</wpt>
<h4 id=nesting-at-scope>
Nested ''@scope'' Rules</h4>
When the ''@scope'' rule is a [=nested group rule=],
an ''&'' in the <<scope-start>> selector
refers to the elements matched
by the nearest ancestor style rule.
<div class=example>
That is, the following code:
<pre highlight=css>
.parent {
color: blue;
@scope (& > .scope) to (& .limit) {
& .content {
color: red;
}
}
}
</pre>
is equivalent to:
<pre highlight=css>
.parent { color: blue; }
@scope (.parent > .scope) to (:where(:scope) .limit) {
:where(:scope) .content {
color: red;
}
}
</pre>
</div>
<div class=note>
The ''&'' selector behaves like <code>:where(:scope)</code>
in ''@scope'' rules.
</div>
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Mixing Nesting Rules and Declarations {#mixing}
-----------------------------------------------
When a style rule contains both declarations
and [=nested style rules=] or [=nested group rules=],
all three can be arbitrarily mixed.
Declarations coming after or between rules
are implicitly wrapped in [=nested declarations rules=],
6D5F
to preserve their order relative to the other rules.
<div class=example>
For example,
in the following code:
<pre class=lang-css>
article {
color: green;
& { color: blue; }
color: red;
}
/* equivalent to */
article { color: green; }
:is(article) { color: blue; }
article { color: red; }
/* NOT equivalent to */
article { color: green; }
article { color: red; }
:is(article) { color: blue; }
</pre>
</div>
For the purpose of determining the [[css-cascade-4#cascade-sort|Order Of Appearance]],
[=nested style rules=] and [=nested group rules=]
are considered to come <em>after</em> their parent rule.
<div>
For example:
<xmp class=lang-css>
article {
color: blue;
& { color: red; }
}
</xmp>
Both declarations have the same specificity (0,0,1),
but the nested rule is considered to come <em>after</em> its parent rule,
so the ''color: red'' declarations wins the cascade.
On the other hand, in this example:
<xmp class=lang-css>
article {
color: blue;
:where(&) { color: red; }
}
</xmp>
The '':where()'' pseudoclass reduces the specificity of the [=nesting selector=] to 0,
so the ''color: red'' declaration now has a specificity of (0,0,0),
and loses to the ''color: blue'' declaration
before "Order Of Appearance" comes into consideration.
</div>
Note: While one <em>can</em> freely intermix declarations and nested rules,
it's harder to read and somewhat confusing to do so,
since the later properties are automatically wrapped in a [=nested declarations rule=]
that doesn't appear in the source text.
For readability's sake,
it's recommended that authors put all their properties first in a style rule,
before any nested rules.
(This also happens to act slightly better in older user agents:
due to specifics of how parsing and error-recovery work,
properties appearing after nested rules can get skipped.)
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Nesting Selector: the ''&'' selector {#nest-selector}
=====================================================
When using a <a>nested style rule</a>,
one must be able to refer to the elements matched by the parent rule;
that is, after all, <em>the entire point of nesting</em>.
To accomplish that,
this specification defines a new selector,
the <dfn export>nesting selector</dfn>,
written as <dfn selector>&</dfn> (U+0026 AMPERSAND).
When used in the selector of a <a>nested style rule</a>,
the <a>nesting selector</a> represents the elements matched by the parent rule.
When used in any other context,
it represents the same elements as '':scope'' in that context
(unless otherwise defined).
<wpt>
top-level-is-scope.html
</wpt>
<div class="note">
The <a>nesting selector</a> can be desugared
by replacing it with the parent style rule's selector,
wrapped in an '':is()'' selector.
For example,
<pre class=lang-css>
a, b {
& c { color: blue; }
}
</pre>
is equivalent to
<pre class=lang-css>
:is(a, b) c { color: blue; }
</pre>
</div>
The [=nesting selector=] cannot represent pseudo-elements
(identical to the behavior of the '':is()'' pseudo-class).
<div class=example>
For example, in the following style rule:
<pre class=lang-css>
.foo, .foo::before, .foo::after {
color: red;
&:hover { color: blue; }
}
</pre>