Overview.bs

<pre class=metadata>
Title: CSS Containment Module Level 3
Level: 3
Shortname: css-contain
Status: ED
Prepare for TR: no
Work Status: Exploring
Group: csswg
ED: https://drafts.csswg.org/css-contain-3/
TR: https://www.w3.org/TR/css-contain-3/
Editor: Tab Atkins, Google, http://xanthir.com/contact/, w3cid 42199
Editor: Florian Rivoal, On behalf of Bloomberg, https://florian.rivoal.net/, w3cid 43241
Editor: Miriam E. Suzanne, Invited Expert, http://miriamsuzanne.com/contact, w3cid 117151
Abstract: This CSS module describes the 'contain' property, which indicates that the element’s subtree is independent of the rest of the page. This enables heavy optimizations by user agents when used well.
Test Suite: https://test.csswg.org/harness/results/css-contain-1_dev/
</pre>
<pre class=link-defaults>
spec:css2; type:dfn; text:stacking context
spec:css2; type:property; text:content
spec:css-display-3; type:property; text:display
spec:css2; type:property; text:counter-increment
spec:css-ui-3; type:property; text:text-overflow
spec:css-grid-1; type:property; text:grid
spec:css-break-3; type:dfn; text:forced break
spec:css-break-3; type:dfn; text:fragmentation
spec:css-break-3; type:dfn; text:fragmentation container
spec:css-break-3; type:dfn; text:fragmentation context
spec:css-break-3; type:dfn; text:fragmented flow
spec:css-contain-2; type:dfn; text:layout containment box
spec:css-contain-2; type:dfn; text:size containment box
spec:css-pseudo-4; type:dfn; text:fictional tag sequence
spec:css-sizing-4; type:property; text:contain-intrinsic-size
spec:css-sizing-4; type:property; text:aspect-ratio
spec:intersection-observer; type:dfn; text:intersection root
spec:css-sizing-3; type:property;
	text:min-width
	text:min-height
spec:css-sizing-3; type:dfn; text:width
spec:css-sizing-3; type:property; text:height
spec:css-sizing-3; type:dfn; text:height
spec:css-sizing-3; type:dfn; text:size
spec:css-values-4; type:dfn; text:relative length
spec:web-animations-1; type:dfn; text:effect values
</pre>

<h2 id='intro'>
Introduction</h2>

	Issue: This is a diff spec over <a href="https://www.w3.org/TR/css-contain-2/">CSS Containment Level 2</a>.
	It is currently an Exploratory Working Draft:
	if you are implementing anything, please use Level 2 as a reference.
	We will merge the Level 2 text into this draft once it reaches CR.

<h3 id="interaction">
Module Interactions</h3>

	This document defines new features not present in earlier specifications.
	In addition, it aims to replace and supersede [[!CSS-CONTAIN-1]]
	once stable.

<h3 id="values">
Value Definitions</h3>

	This specification follows the <a href="https://www.w3.org/TR/CSS2/about.html#property-defs">CSS property definition conventions</a> from [[!CSS2]]
	using the <a href="https://www.w3.org/TR/css-values-3/#value-defs">value definition syntax</a> from [[!CSS-VALUES-3]].
	Value types not defined in this specification are defined in CSS Values &amp; Units [[!CSS-VALUES-3]].
	Combination with other CSS modules may expand the definitions of these value types.

	In addition to the property-specific values listed in their definitions,
	all properties defined in this specification
	also accept the <a>CSS-wide keywords</a> as their property value.
	For readability they have not been repeated explicitly.

<h2 id='contain-property'>
Strong Containment: the 'contain' property</h2>

	Issue: [[css-contain-2#contain-property]]

	<pre class='propdef partial'>
		Name: contain
		New Values: layout || style || paint || [ size | inline-size ]
	</pre>

	<dl dfn-type=value dfn-for=contain>
		<dt><dfn>inline-size</dfn>
		<dd>
			This value turns on [=inline-size containment=] for the element.
			This prevents the [=inline-size=] of its [=principal box=]
			from directly depending on its contents.

			Note: There can still be indirect dependencies,
			see [[#containment-inline-size]].
	</dl>


<h2 id='containment-types'>
Types of Containment</h2>

	Issue: [[css-contain-2#containment-types]]

<h3 id='containment-inline-size'>
Inline-Size Containment</h3>

	Giving an element <dfn export>inline-size containment</dfn>
	applies [=size containment=] to the [=inline-axis=] sizing of its [=principal box=].
	This means the [=inline-axis=] [=intrinsic sizes=] of the [=principal box=]
	are determined as if the element had no content.
	However, content continues to impact the box’s [=block-axis=] [=intrinsic sizes=] as usual,
	and the box is allowed to [=fragmentation|fragment=] normally in the [=block axis=].

	<div class=note>
		<span class="marker">Note:</span> In some cases,
		a box’s [=block-axis=] [=intrinsic sizes=]
		can impact layout in the parent [=formatting context=]
		in ways that affect the box’s [=inline size=]
		(e.g. by triggering scrollbars on an ancestor element),
		creating a dependency of the box’s [=inline size=] on its own content.
		If this changed [=inline size=] results in a different [=block size=],
		that new [=block size=] can loop into further impacting the parent formatting context,
		but not in a way that reverts it to the previously-problematic layout.

		For example, if scrollbars were introduced,
		they are not then removed,
		even if the consequent [=block size=] is small enough to not need them;
		or if a box’s logical height collides with a lower-placed float and is cleared down
		to where it also has more available inline space
		and thus becomes short enough to not have collided,
		it is not them moved back up to its previous problematic size and position.

		Thus, although [=inline-size containment=] prevents
		the box’s content from directly affecting its [=inline size=]
		through its [=inline-axis=] [=intrinsic sizes=],
		its [=inline size=] can still indirectly depend on its contents
		by their effect on its [=block size=].
	</div>

	ISSUE:
		In general, the relationship between an element's inline size
		and it's block size
		is unpredictable and non-monotonic,
		with the block size capable of shifting up and down arbitrarily
		as the inline size is changed.
		Infinite cycles are prevented
		by ensuring that layout does not revert to a previous (known-problematic) state,
		even if a naive analysis of the constraints would allow for such;
		in other words, layout always “moves forward”.
		We believe that current CSS layout specifications incorporate such rules,
		but to the extent that they don't,
		please <a href="https://github.com/w3c/csswg-drafts/issues">inform the CSSWG</a>
		so that these errors can be corrected.

	<div class=example>
		Consider this example,
		where float placement creates a dependency of block sizes on inline sizes:

		<xmp class=lang-markup>
			<section style="width: 200px; border: solid; display: flow-root;">
			  <!-- floated elements that impact the available space -->
			  <div style="float: left; width: 50px; height: 80px; background: blue;"></div>
			  <div style="float: right; width: 50px; height: 80px; background: blue;"></div>
			  <div style="float: left; width: 160px; height: 80px; background: navy;"></div>

			  <!-- parent layout, determining context -->
			  <article style="border: solid orangered; display: flow-root; min-width: min-content">
			    <div style="background: orange; aspect-ratio: 1/1;">
			      Article
			    </div>
			  </article>
			</section>
		</xmp>

		<figure style="float: left; margin: 1em 0.5em">
			<section style="width: 200px; border: solid; display: flow-root;">
			  <!-- floated elements that impact the available space -->
			  <div style="float: left; width: 50px; height: 80px; background: blue;"></div>
			  <div style="float: right; width: 50px; height: 80px; background: blue;"></div>
			  <div style="float: left; width: 160px; height: 80px; background: navy;"></div>

			  <!-- parent layout, determining context -->
			  <article style="border: solid orangered; display: flow-root; min-width: 50px">
			    <div style="background: orange; aspect-ratio: 1/1;">
			      Article
			    </div>
			  </article>
			</section>
		</figure>

		The block layout algorithm will first place the floating boxes,
		with the first two sitting in the left and right corners of the container,
		and the third, being too wide to fit between, being pushed below them.

		The following <code>article</code> will then be laid out.
		Because it is ''display: flow-root'',
		it cannot intersect any floats,
		and thus must take them into account
		when figuring out how to size and position itself.

		The layout engine first attempts to place the <code>article</code>
		flush with the top of the container,
		resulting a ''100px'' width,
		plenty wide enough to accommodate its [=min-content size=].
		However, due the 'aspect-ratio' of its child,
		this would cause the <code>article</code> to be ''100px'' tall as well,
		which would intersect the third float 80px below,
		so this layout opportunity is discarded.

		It then attempts to position the <code>article</code>
		flush with the top of the third float,
		in the narrow ''40px''-wide space to its right.
		However, since the <code>article</code>’s 'min-width' makes it too large
		to fit in the 40px-wide space beside the third float,
		it shifts below that one as well,
		forming a 200px square below all the floated boxes.

		<figure style="float: right; margin: 1em 0.5em">
			<section style="width: 200px; border: solid; display: flow-root;">
			  <!-- floated elements that impact the available space -->
			  <div style="float: left; width: 50px; height: 80px; background: blue;"></div>
			  <div style="float: right; width: 50px; height: 80px; background: blue;"></div>
			  <div style="float: left; width: 160px; height: 80px; background: navy;"></div>

			  <!-- parent layout, determining context -->
			  <article style="border: solid orangered; display: flow-root;">
			    <div style="background: orange; aspect-ratio: 1/1;">
			      Article
			    </div>
			  </article>
			</section>
		</figure>

		If the 'min-width' is removed from the <code>article</code>,
		or if [=inline-size containment=] is added to
		either the <code>article</code> or <code>header</code>
		(causing ''min-width: min-content'' to resolve to zero),
		then the <code>article</code> will fit as a 40px square
		next to the final floated <code>div</code>
		(possibly with some of its content overflowing).

		At this point, the width and height of the <code>article</code>
		(''40px'' each)
		<em>would</em> fit back in the first considered space,
		flush with the top of the container.
		However, the box is not returned to the previous position,
		because the layout engine knows already
		that this position would result in an invalid layout.
	</div>

	Giving an element [=inline-size containment=]
	has no effect if any of the following are true:

	* if the element does not generate a <a>principal box</a>
		(as is the case with ''display: contents'' or ''display: none'')
	* if its [=inner display type=] is ''display/table''
	* if its [=principal box=] is
		an <a spec="css-display-3">internal table box</a>
	* if its [=principal box=] is
		an <a spec="css-display-3">internal ruby box</a>
		or a <a spec="css-display-3" lt="atomic inline">non-atomic</a> <a spec="css-display-3">inline-level</a> box


<h2 id="container-queries">
Container Queries</h2>

	While [=media queries=] provide a method to query
	aspects of the user agent or device environment
	that a document is being displayed in
	(such as viewport dimensions or user preferences),
	[=container queries=] allow testing aspects of elements within the document
	(such as box dimensions or computed styles).

	By default, all elements are [=query containers=]
	for the purpose of [=container style queries=],
	and can be established as [=query containers=]
	for [=container size queries=] by specifying
	the additional query types using the 'container-type' property
	(or the 'container' [=shorthand=]).
	Style rules applying to a [=query container=]’s <a>shadow-including descendants</a>
	can be conditioned by querying against it,
	using the ''@container'' [=conditional group rule=].

	<div class=example>
		For example, we can define the main content area and sidebar as containers,
		and then describe a ''.media-object'' that changes
		from vertical to horizontal layout depending on the size of its container:

		<pre class=lang-css>
		main, aside {
		  container: my-layout / inline-size;
		}

		.media-object {
		  display: grid;
		  grid-template: 'img' auto 'content' auto / 100%;
		}

		@container my-layout (inline-size > 45em) {
		  .media-object {
		    grid-template: 'img content' auto / auto 1fr;
		  }
		}
		</pre>

		Media objects in the main and sidebar areas
		will each respond to their own container context.
	</div>

	For selectors with pseudo elements, query containers can be established by
	the <a>shadow-including inclusive ancestors</a> of the <a>ultimate
	originating element</a>.

	<div class=note>
		It follows that:

		* Pseudo elements themselves can not be query containers
		* ''::before'', ''::after'', ''::marker'', and ''::backdrop'' query their
			originating elements
		* ''::first-letter'' and ''::first-line'' query their originating elements,
			even if the <a>fictional tag sequence</a> may push the
			<code>::first-line</code> past other elements for the purpose of
			inheritance and rendering
		* Multiple pseudo elements do not allow pseudo elements to be query
			containers for other pseudo elements. E.g., the host, but not the
			<code>::part()</code>, can be the query container for
			<code>::before</code> in <code>host::part()::before</code>. Similarly,
			<code>::before</code> can not be the query container for the
			<code>::marker</code> in <code>div::before::marker</code>
		* ''::slotted()'' selectors can query containers inside the shadow tree,
			including the slot itself
		* ''::part()'' selectors can query its originating host, but not internal
			query containers inside the shadow tree
		* ''::placeholder'' and ''::file-selector-button'' can query the input
			element, but do not expose any internal containers if the input element is
			implemented using a shadow tree
	</div>

	<div class=example>
		A ::before selector querying the size of the originating element:

		<pre class=lang-html>
		&lt;style>
		  #container {
		    width: 100px;
		    container-type: inline-size;
		  }
		  @container (inline-size < 150px) {
		    #inner::before {
		      content: "BEFORE";
		    }
		  }
		&lt;/style>
		&lt;div id=container>
		  &lt;span id=inner>&lt;/span>
		&lt;/div>
		</pre>
	</div>

	<div class=example>
		A ::slotted() selector for styling a shadow host child can query a
		container in the shadow tree:

		<pre class=lang-html>
		&lt;div id=host style="width:200px">
		  &lt;template shadowroot=open>
		    &lt;style>
		      #container {
		        width: 100px;
		        container-type: inline-size;
		      }
		      @container (inline-size < 150px) {
		        ::slotted(span) {
		          color: green;
		        }
		      }
		    &lt;/style>
		    &lt;div id=container>
		      &lt;slot />
		    &lt;/div>
		  &lt;/template>
		  &lt;span id=slotted>Green&lt;/span>
		&lt;/div>
		</pre>
	</div>

<h3 id="container-type">
Creating Query Containers: the 'container-type' property</h3>

	<pre class='propdef'>
		Name: container-type
		Value: normal || [ size | inline-size ]
		Initial: normal
		Inherited: no
		Applies to: all elements
		Computed value: the keyword ''container-type/normal'' or one or more of ''container-type/size'', ''container-type/inline-size''
		Animation type: not animatable
	</pre>

	The 'container-type' property establishes the element
	as a <dfn export>query container</dfn>
	for the purpose of [=container queries=] that require explicit containment
	(such as [=container size queries=]),
	allowing [=style rules=] styling its descendants
	to query various aspects of its sizing and layout,
	and respond accordingly.

	Unless otherwise noted,
	all elements are [=query containers=]
	for the purpose [=container queries=]
	that do no require explicit containment
	(such as [=container style queries=]),
	reagardless of the specified 'container-type'.

	Values have the following meanings:

	<dl dfn-type=value dfn-for=container-type>
		<dt><dfn>size</dfn>
		<dd>
			Establishes a [=query container=] for [=container size queries=]
			on both the [=inline axis|inline=] and [=block axis=].
			Applies [=layout containment=],
			[=style containment=],
			and [=size containment=]
			to the [=principal box=].
		<dt><dfn>inline-size</dfn>
		<dd>
			Establishes a [=query container=] for [=container size queries=]
			on the container’s own [=inline axis=].
			Applies [=layout containment=],
			[=style containment=],
			and [=inline-size containment=]
			to the [=principal box=].
		<dt><dfn>normal</dfn>
		<dd>
			The element is not a [=query container=]
			for any [=container size queries=],
			but remains a [=query container=] for [=container style queries=].
	</dl>

	<div class=example>
		For example, authors can create container-responsive typography,
		adjusting 'font-size', 'line-height', and other typographic concerns
		based on the size of a container:

		<pre class=lang-css>
		aside, main {
		  container-type: inline-size;
		}

		h2 { font-size: 1.2em; }

		@container (width > 40em) {
		  h2 { font-size: 1.5em; }
		}
		</pre>

		The ''40em'' value used in the query condition
		is relative to the [=computed value=] of 'font-size'
		on the relevant [=query container=].
	</div>

	<div class=example>
		Containers can also expose computed style values for querying.
		This can be useful for toggling behavior across multiple properties:

		<pre class=lang-css>
		section {
		  container-type: style;
		}

		@container (--cards: small) {
		  article {
		    border: thin solid silver;
		    border-radius: 0.5em;
		    padding: 1em;
		  }
		}
		</pre>
	</div>


<h3 id="container-name">
Naming Query Containers: the 'container-name' property</h3>

	<pre class='propdef'>
		Name: container-name
		Value: none | <<custom-ident>>+
		Initial: none
		Inherited: no
		Applies to: all elements
		Computed Value: the keyword ''container-name/none'', or an ordered list of [=identifiers=]
		Animation type: not animatable
	</pre>

	The 'container-name' property
	specifies a list of <dfn export lt="query container name">query container names</dfn>.
	These names can be used by ''@container'' rules
	to filter which [=query containers=] are targeted.

	<dl dfn-for=container-name dfn-type=value>
	<dt><dfn>none</dfn>
	<dd>
		The [=query container=] has no [=query container name=].
	<dt><dfn><<custom-ident>></dfn>
	<dd>
		Specifies a [=query container name=] as an [=identifier=].
		The keywords ''container-name/none'', ''and'', ''not'', and ''or'' are excluded from this <<custom-ident>>.
	</dl>

	<div class=example>
		In some cases, we want to query aspects of a specific container,
		even if it’s not the nearest ancestor container.
		For example, we might want to query the height of a main content area,
		and the width of a more nested inline-container.

		<pre class=lang-css>
		main {
		  container-type: size;
		  container-name: my-page-layout;
		}

		.my-component {
		  container-type: inline-size;
		  container-name: my-component-library;
		}

		@container my-page-layout (block-size > 12em) {
		  .card { margin-block: 2em; }
		}

		@container my-component-library (inline-size > 30em) {
		  .card { margin-inline: 2em; }
		}
		</pre>
	</div>


<h3 id="container-shorthand">
Creating Named Containers: the 'container' shorthand</h3>

	<pre class='propdef shorthand'>
		Name: container
		Value: <<'container-name'>> [ / <<'container-type'>> ]?
	</pre>

	The 'container' [=shorthand property=] sets
	both 'container-type' and 'container-name' in the same declaration.
	If <<'container-type'>> is omitted,
	it is reset to its [=initial value=].

	<div class=example>
		We can define both a 'container-type' and 'container-name'
		using the shorthand syntax:

		<pre class=lang-css>
		main {
		  container: my-layout / size;
		}

		.grid-item {
		  container: my-component / inline-size;
		}
		</pre>
	</div>


<h3 id="container-rule">
Container Queries: the ''@container'' rule</h3>

	The <dfn at-rule id="at-ruledef-container">@container</dfn> rule
	is a [=conditional group rule=] whose condition is
	a <dfn export>container query</dfn>,
	which is a boolean combination of [=container size queries=] and/or [=container style queries=].
	Style declarations within the <<stylesheet>> block of an ''@container'' rule
	are [[css-cascade-4#filtering|filtered]] by its condition
	to only match when the [=container query=]
	is true for their element’s [=query container=].

	The syntax of the ''@container'' rule is:

	<pre class="prod def">
	@container [ <<container-name>> ]? <<container-condition>> {
	  <<stylesheet>>
	}
	</pre>

	where:

	<pre class="prod def">
	<dfn><<container-name>></dfn> = <<custom-ident>>
	<dfn><<container-condition>></dfn> = not <<query-in-parens>>
	                      | <<query-in-parens>> [ [ and <<query-in-parens>> ]* | [ or <<query-in-parens>> ]* ]
	<dfn><<query-in-parens>></dfn>     = ( <<container-condition>> )
	                      | ( <<size-feature>> )
	                      | style( <<style-query>> )
	                      | <<general-enclosed>>

	<dfn><<style-query>></dfn>         = <<style-condition>> | <<style-feature>>
	<dfn><<style-condition>></dfn>     = not <<style-in-parens>>
	                      | <<style-in-parens>> [ [ and <<style-in-parens>> ]* | [ or <<style-in-parens>> ]* ]
	<dfn><<style-in-parens>></dfn>     = ( <<style-condition>> )
	                      | ( <<style-feature>> )
	                      | <<general-enclosed>>
	</pre>

	The keywords ''container-name/none'', ''and'', ''not'', and ''or''
	are excluded from the <<custom-ident>> above.

	For each element,
	the [=query container=] to be queried
	is selected from among the element’s ancestor [=query containers=]
	that are established as a valid [=query container=]
	for all the [=container features=]
	in the <<container-condition>>.
	The optional <<container-name>>
	filters the set of [=query containers=] considered
	to just those with a matching [=query container name=].

	Once an eligible [=query container=] has been selected for an element,
	each [=container feature=] in the <<container-condition>>
	is evaluated against that [=query container=].
	If no ancestor is an eligible [=query container=],
	then the [=container query=] is ''unknown'' for that element.

	<div class=example>
		As with [=media queries=],
		we can string together multiple conditions in a single query list:

		<pre class=lang-css>
		@container card (inline-size > 30em) and style(--responsive: true) {
		  /* styles */
		}
		</pre>

		The styles above will only be applied
		if there is an ancestor container named "card"
		that meets both the '@container/inline-size'
		and [=container style query|style=] conditions.
	</div>

	Style rules defined on an element inside multiple nested [=container queries=]
	apply when all of the wrapping [=container queries=] are true for that element.

	Note: Nested [=container queries=] can evaluate in relation to different containers,
	so it is not always possible to merge the individual <<container-condition>>s
	into a single query.

	<div class=example>
		While it is not possible to query multiple containers
		in a single [=container query=],
		that can be achieved by nesting multiple queries:

		<pre class=lang-css>
		@container card (inline-size > 30em) {
		  @container style(--responsive: true) {
		    /* styles */
		  }
		}
		</pre>

		The styles above will only be applied
		if there is an ancestor container named "card"
		that meets the '@container/inline-size' condition,
		as well as an ancestor container
		meeting [=container style query|style=] condition.
	</div>

	Global, name-defining [=at-rules=]
	such as ''@keyframes'' or ''@font-face'' or ''@layer''
	that are defined inside [=container queries=]
	are not constrained by the [=container query=] conditions.


<h3 id="animated-containers">
Animated Containers</h3>

	A change in the evaluation of a [=container query=] must be part of a [=style change event=],
	even when the change occurred because of [=effect values|animation effects=].

	<div class=example>
		A transition on a sibling element can indirectly affect the size of a
		container, triggering [=style change events=] whenever container queries
		change their evaluation as a result:

		<pre class=lang-css>
			main {
			  display: flex;
			  width: 300px;
			}

			#container {
			  container-type: inline-size;
			  flex: 1;
			}

			/* Resolved width is initially 200px, but changes as the transition
			   on #sibling progresses. */
			#inner {
			  transition: 1s background-color;
			  background-color: tomato;
			}

			/* When this container query starts (or stops) applying, a transition
			   must start on background-color on #inner. */
			@container (width <= 150px) {
			  #inner {
			    background-color: skyblue;
			  }
			}

			#sibling {
			  width: 100px;
			  transition: width 1s;
			}

			#sibling:hover {
			  width: 200px;
			}
		</pre>

		<pre class=lang-html>
			&lt;main>
			  &lt;div id=container>
			    &lt;div id=inner>Inner&lt;/div>
			  &lt;/div>
			  &lt;div id=sibling>Sibling&lt;/div>
			&lt;/main>
		</pre>

	</div>

	Changes in [=computed values=] caused by [=container query length=] units
	must also be part of a [=style change event=].


<h2 id="container-features">
Container Features</h2>

	A <dfn export>container feature</dfn>
	queries a specific aspect of a [=query container=].


<h3 id="size-container">
Size Container Features</h3>

	A <dfn export>container size query</dfn>
	allows querying
	the size of the [=query container=]’s [=principal box=].
	It is a boolean combination of
	individual <dfn>size features</dfn> (<<size-feature>>)
	that each query a single, specific dimensional feature of the [=query container=].
	The syntax of a <dfn><<size-feature>></dfn> is the same as for a [=media feature=]:
	a feature name, a comparator, and a value.
	[[mediaqueries-5]]
	The boolean syntax and logic combining [=size features=] into a [=container size query|size query=]
	is the same as for [=CSS feature queries=].
	(See ''@supports''. [[!CSS-CONDITIONAL-3]])

	If the [=query container=] does not have a [=principal box=],
	or the principal box is not a [=layout containment box=],
	or the [=query container=] does not support [=container size queries=] on the relevant axes,
	then the result of evaluating the [=size feature=] is unknown.

	[=Relative length=] units in [=container query=] conditions
	are evaluated based on the the [=computed values=] of the [=query container=].

	Note: This is different from the handling of relative units in [=media queries=].

	<div class=example>
		For example, [=query containers=] with different font-sizes
		will evaluate ''em''-based queries relative to their own font sizes:

		<pre class=lang-css>
		aside, main {
		  container-type: inline-size;
		}

		aside { font-size: 16px; }
		main { font-size: 24px; }

		@container (width > 40em) {
		  h2 { font-size: 1.5em; }
		}
		</pre>

		The ''40em'' value used in the query condition
		is relative to the [=computed value=] of 'font-size'
		on the relevant [=query container=]:

		* For any ''h2'' inside ''aside'',
			the query condition will be true above ''640px''.
		* For any ''h2'' inside ''main'',
			the query condition will be true above ''960px''.
	</div>

<h4 id="width">
Width: the '@container/width' feature</h4>

	<pre class="descdef mq">
		Name: width
		For: @container
		Value: <<length>>
		Type: range
	</pre>

	The '@container/width' [=container feature=]
	queries the [=width=]
	of the [=query container=]’s [=content box=].

<h4 id="height">
Height: the '@container/height' feature</h4>

	<pre class="descdef mq">
		Name: height
		For: @container
		Value: <<length>>
		Type: range
	</pre>

	The '@container/height' [=container feature=]
	queries the [=height=]
	of the [=query container=]’s [=content box=].

<h4 id="inline-size">
Inline-size: the '@container/inline-size' feature</h4>

	<pre class="descdef mq">
		Name: inline-size
		For: @container
		Value: <<length>>
		Type: range
	</pre>

	The '@container/inline-size' [=container feature=]
	queries the [=size=]
	of the [=query container=]’s [=content box=]
	in the [=query container=]’s [=inline axis=].


<h4 id="block-size">
Block-size: the '@container/block-size' feature</h4>

	<pre class="descdef mq">
		Name: block-size
		For: @container
		Value: <<length>>
		Type: range
	</pre>

	The '@container/block-size' [=container feature=]
	queries the [=size=]
	of the [=query container=]’s [=content box=]
	in the [=query container=]’s [=block axis=].


<h4 id="aspect-ratio">
Aspect-ratio: the '@container/aspect-ratio' feature</h4>

	<pre class="descdef mq">
		Name: aspect-ratio
		For: @container
		Value: <<ratio>>
		Type: range
	</pre>

	The '@container/aspect-ratio' [=container feature=] is defined as the ratio
	of the value of the '@container/width' [=container feature=]
	to the value of the '@container/height' [=container feature=].


<h4 id="orientation">
Orientation: the '@container/orientation' feature</h4>

	<pre class="descdef mq">
		Name: orientation
		For: @container
		Value: portrait | landscape
		Type: discrete
	</pre>

	<dl dfn-type=value dfn-for="@container/orientation">
		<dt><dfn>portrait</dfn>
		<dd>
			The '@container/orientation' [=container feature=] is ''portrait''
			when the value of the '@container/height' [=container feature=]
			is greater than or equal to
			the value of the '@container/width' [=container feature=].
		<dt><dfn>landscape</dfn>
		<dd>Otherwise '@container/orientation' is ''landscape''.
	</dl>


<h3 id="style-container">
Style Container Features</h3>

	A <dfn export>container style query</dfn>
	allows querying
	the [=computed values=] of the [=query container=].
	It is a boolean combination of
	individual <dfn>style features</dfn> (<<style-feature>>)
	that each query a single, specific property of the [=query container=].
	The syntax of a <dfn><<style-feature>></dfn> is the same as for a [=declaration=]
	[[!CSS-SYNTAX-3]],
	and its query is true if
	the [=computed value=] of the given property on the [=query container=]
	matches the given value
	(which is also [=computed value|computed=] with respect to the [=query container=]),
	unknown if the property or its value is invalid or unsupported,
	and false otherwise.
	The boolean syntax and logic combining [=style features=] into a [=container style query|style query=]
	is the same as for [=CSS feature queries=].
	(See ''@supports''. [[!CSS-CONDITIONAL-3]])

	[=Style features=] that query a [=shorthand property=] are true if the
	[=computed values=] match for each of its [=longhand properties=],
	and false otherwise.

	[=Cascade-dependent keywords=], such as ''revert'' and ''revert-layer'',
	are invalid as values in a [=style feature=], and cause the
	[=container style query=] to be false.

	Note: The remaining non-cascade-dependent [=CSS-wide keywords=]
	are [=computed value|computed=] with respect to the [=query container=],
	the same as other values.

<h2 id="container-lengths">
Container Relative Lengths: the ''cqw'', ''cqh'', ''cqi'', ''cqb'', ''cqmin'', ''cqmax'' units</h2>

	<dfn lt="container query length">Container query length units</dfn>
	specify a length relative to the dimensions of a [=query container=].
	Style sheets that use [=container query length=] units can more easily move components
	from one [=query container=] to another.

	The [=container query length=] units are:

	<table class="data">
	<caption>Informative Summary of Container Units</caption>
	<thead>
		<tr><th>unit<th>relative to
	</thead>
	<tbody>
		<tr><td>''cqw''
			<td>1% of a [=query container=]’s [=width=]
		<tr><td>''cqh''
			<td>1% of a [=query container=]’s [=height=]
		<tr><td>''cqi''
			<td>1% of a [=query container=]’s [=inline size=]
		<tr><td>''cqb''
			<td>1% of a [=query container=]’s [=block size=]
		<tr><td>''cqmin''
			<td>The smaller value of ''cqi'' or ''cqb''
		<tr><td>''cqmax''
			<td>The larger value of ''cqi'' or ''cqb''
	</tbody>
	</table>

	For each element,
	[=container query length=] units are evaluated
	as [=container size queries=] on the relevant axis (or axes)
	described by the unit.
	The [=query container=] for each axis
	is the nearest ancestor container
	that accepts [=container size queries=] on that axis.
	If no eligible [=query container=] is available,
	then use the [=small viewport size=] for that axis.

	Note: In some cases ''cqi'' and ''cqb'' units on the same element
	will evaluate in relation to different [=query containers=].
	Similarly, ''cqmin'' and ''cqmax'' units represent
	the larger or smaller of the ''cqi'' and ''cqb'' units,
	even when those dimensions come from different [=query containers=].

	Child elements do not inherit the relative values as specified for their parent;
	they inherit the <a>computed values</a>.

	<div class=example>
		Authors can ensure that [=container query length=] units
		have an appropriate [=query container=]
		by applying them inside a [=container query=]
		that relies on the same ''container-type''.
		Custom fallback values can be defined outside the [=container query=]:

		<pre class=lang-css>
		/* The fallback value does not rely on containment */
		h2 { font-size: 1.2em; }

		@container (inline-size >= 0px) {
		  /* only applies when an inline-size container is available */
		  h2 { font-size: calc(1.2em + 1cqi); }
		}
		</pre>
	</div>


<h2 id="apis">APIs</h2>


<h3 id="the-csscontainerrule-interface">
The <code>CSSContainerRule</code> interface</h3>

	The {{CSSContainerRule}} interface represents a ''@container'' rule.

	<pre class='idl'>
	[Exposed=Window]
	interface CSSContainerRule : CSSConditionRule {
	};
	</pre>

	<dl class='idl-attributes'>
		<dt><code>conditionText</code> of type <code>CSSOMString</code> (CSSContainerRule-specific definition for attribute on CSSConditionRule)
		<dd>The <code>conditionText</code> attribute (defined on the <code>CSSConditionRule</code> parent rule),
			on getting, must return the condition that was specified,
			without any logical simplifications,
			so that the returned condition will evaluate to the same result
			as the specified condition
			in any conformant implementation of this specification
			(including implementations that implement future extensions
			allowed by the <<general-enclosed>> extensibility mechanism in this specification).
			In other words,
			token stream simplifications are allowed
			(such as reducing whitespace to a single space
			or omitting it in cases where it is known to be optional),
			but logical simplifications (such as removal of unneeded parentheses,
			or simplification based on evaluating results) are not allowed.
	</dl>

	Issue(7033): Add CSSOM API for CSSContainerRule

	Issue(6205): Container Queries should have a <code>matchContainer</code> method.
	This will be modeled on {{matchMedia()}} and the {{MediaQueryList}} interface,
	but applied to Elements rather than the Window.
	When measuring layout sizes, it behaves Similar to <code>resizeObserver</code>,
	but it provides the additional Container Query syntax and features.

Suppressing An Element’s Contents Entirely: the 'content-visibility' property {#content-visibility}
=================

	Issue: [[css-contain-2#content-visibility]]

Privacy and Security Considerations {#priv-sec}
==============================================

	Issue: [[css-contain-2#priv-sec]]

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<h2 class="no-num non-normative" id="changes">Appendix A. Changes</h2>

This appendix is <em>informative</em>.

<h3 id="changes-2021-12">
Changes since the 21 December 2021 First Public Working Draft</h3>

	Significant changes since the <a href="https://www.w3.org/TR/2021/WD-css-contain-3-20211221/">21 December 2021 First Public Working Draft</a> include:

	* Allow the computed value of 'container-name' to include duplicate identifiers.
		(<a href="https://github.com/w3c/csswg-drafts/issues/7181">Issue 7181</a>)
	* Make the <<'container-name'>> in the 'container' shorthand required.
		(<a href="https://github.com/w3c/csswg-drafts/issues/7142">Issue 7142</a>)
	* Clarify handling of [=shorthand properties=] in [=container style queries=].
		(<a href="https://github.com/w3c/csswg-drafts/issues/7095">Issue 7095</a>)
	* [=Cascade-dependent keywords=] are not allowed as values in a
		[=style feature=], and cause the [=container style query=] to be false.
		(<a href="https://github.com/w3c/csswg-drafts/issues/7080">Issue 7080</a>)
	* Change the initial value of 'container-type' to be ''style''.
		(<a href="https://github.com/w3c/csswg-drafts/issues/6393">Issue 6393</a>)
	* Remove the block-size value from 'container-type',
		since single-axis block-size containment is not currently possible.
		(<a href="https://github.com/w3c/csswg-drafts/issues/1031">Issue 1031</a>)
	* Remove the <<string>> option from the 'container-name' syntax.
		Container names must be <<custom-ident>>s.
		(<a href="https://github.com/w3c/csswg-drafts/issues/6405">Issue 6405</a>)
	* Reverse the order of <<'container-name'>> and <<'container-type'>>
		in the 'container' shorthand property, with both being optional.
		(<a href="https://github.com/w3c/csswg-drafts/issues/6393">Issue 6393</a>)
	* Allow <<general-enclosed>> syntax in <<container-condition>>s,
		for the sake of forward compatability.
		(<a href="https://github.com/w3c/csswg-drafts/issues/6396">Issue 6396</a>)
	* Remove the size function syntax from <<size-feature>> queries.
		(<a href="https://github.com/w3c/csswg-drafts/issues/6870">Issue 6870</a>)
	* Update the [=query container=] selection process
		to account for necessary container-types,
		and removed the explicit type-selection syntax.
		(<a href="https://github.com/w3c/csswg-drafts/issues/6644">Issue 6644</a>)
	* Remove state query features, which have been deferred.
		(<a href="https://github.com/w3c/csswg-drafts/issues/6402">Issue 6402</a>)
	* Clarify container selection around pseudo-elements and the shadow-DOM.
		(<a href="https://github.com/w3c/csswg-drafts/issues/5984">Issue 5984</a>
		and <a href="https://github.com/w3c/csswg-drafts/issues/6711">Issue 6711</a>)

<h3 id="l3-changes">
Changes from <a href="https://www.w3.org/TR/css-contain-2/">CSS Containment Level 2</a>
</h3>

	* Introduces [=inline-size containment=].
	* Defines the terms, properties, units, and at-rule
		needed for [=Container Queries=]

ISSUE: [[css-contain-2#changes]]

<h2 class="no-num" id="acknowledgments">Acknowledgments</h2>

	Comments and previous work from
	Adam Argyle,
	Amelia Bellamy-Royds,
	Anders Hartvoll Ruud,
	Brian Kardell,
	Chris Coyier,
	Christopher Kirk-Nielsen,
	David Herron,
	Elika J. Etemad (fantasai),
	Eric Portis,
	Ethan Marcotte,
	Geoff Graham,
	Gregory Wild-Smith,
	Ian Kilpatrick,
	Jen Simmons,
	Kenneth Rohde Christiansen,
	L. David Baron,
	Lea Verou,
	Martin Auswöger,
	Martine Dowden,
	Mike Riethmuller,
	Morten Stenshorne,
	Nicole Sullivan,
	Rune Lillesveen,
	Scott Jehl
	Scott Kellum,
	Stacy Kvernmo,
	Theresa O’Connor,
	Una Kravets,
	and many others have contributed to this specification.