Overview.bs

<pre class="metadata">
Title: CSS Overflow Module Level 3
Status: ED
Work Status: Revising
ED: https://drafts.csswg.org/css-overflow-3/
Shortname: css-overflow
Group: csswg
Level: 3
TR: https://www.w3.org/TR/css-overflow-3/
Previous version: https://www.w3.org/TR/2021/WD-css-overflow-3-20211223/
Former Editor: L. David Baron, Mozilla https://www.mozilla.org/, https://dbaron.org/, w3cid 15393
Editor: Elika J. Etemad / fantasai, Invited Expert, http://fantasai.inkedblade.net/contact, w3cid 35400
Editor: Florian Rivoal, On behalf of Bloomberg, http://florian.rivoal.net/, w3cid 43241
Abstract: This module contains the features of CSS relating to scrollable overflow handling in visual media.
	This level is focused on completing a precise specification for the existing overflow features,
	including the 'overflow' property and its longhands;
	'line-clamp', its longhands, and its legacy pre-standard syntax;
	and the 'text-overflow' property.
	A few additional features introduced in support of [[CSS-CONTAIN-1]] and [[CSS-CONTAIN-2]] are also defined:
	''overflow: clip'' and the 'overflow-clip-margin' property.
Ignored Vars: B, P
Ignored Terms:viewport, flex order, -webkit-box-orient
At Risk: the 'max-lines' property
Status Text:
	The description of 'overflow' and its longhands is considered
	significantly more complete and correct than previous working drafts
	or than [[CSS2]],
	but a few questions and issues remain open.
	''overflow: clip' and 'overflow-clip-margin' are rather new,
	and lack implementation experience.
	'text-overflow' is stable,
	is unchanged form its earlier definition in [[CSS-UI-3]].
	While not yet fully validated by implementation experience,
	the design of 'line-clamp' and its longhands is considered roughly complete.
	Earlier versions of this specification included experimental new ideas
	for handling overflow by creating new boxes using fragmentation.
	These ideas are not abandoned; they are merely <a href="https://drafts.csswg.org/css-overflow-4/">deferred until Level 4</a>.
	Work will resume on fragmented overflow once this level stabilizes completed.
</pre>
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<pre class="link-defaults">
spec:css-transforms-2; type:property; text:transform-style
type: dfn; spec:css-multicol-1; text:overflow column
type: dfn; spec:css-transforms-2; text:3d rendering context
spec:css-writing-modes-4; type:dfn; text:end
spec:css-pseudo-4; type:selector; text:::first-line
spec:css-pseudo-4; type:selector; text:::first-letter
spec:css-display-3; type:property; text:display
spec:css-sizing-3; type:property; text:max-height
spec:css-box-4; type:dfn; text:content area
spec:mediaqueries-3; type:value; text:print
spec:html; type:method; text:focus()
</pre>
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<pre class="anchors">
url: https://www.w3.org/TR/2008/CR-css3-marquee-20081205/#the-overflow-style; type: property; text: overflow-style;
url: https://www.w3.org/TR/CSS22/visuren.html#positioned-element; type: dfn; spec:css2; text:positioned;
url: https://www.w3.org/TR/CSS22/visudet.html#strut-element; type: dfn; spec:css2; text:strut;
</pre>
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<h2 id="intro">
Introduction</h2>

	In CSS Level 1 [[CSS1]], placing more content than would fit
	inside an element with a specified size
	was generally an authoring error.
	Doing so caused the content to extend
	outside the bounds of the element,
	which would likely cause
	that content to overlap with other elements.

	CSS Level 2 [[CSS2]] introduced the 'overflow' property,
	which allows authors to have overflow be handled by scrolling,
	which means it is no longer an authoring error.
	It also allows authors to specify
	that overflow is handled by clipping,
	which makes sense when the author's intent
	is that the content not be shown.

	This specification introduces the long-standing de-facto 'overflow-x' and 'overflow-y' properties,
	adds a ''overflow/clip'' value,
	and defines overflow handling more fully.

	[Something something 'max-lines'.]

	Note: This specification also reproduces the definition of the 'text-overflow' property
	previously defined in [[CSS-UI-3]],
	with no addition or modification,
	in order to present 'text-overflow' and 'block-ellipsis' together.

<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.

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

	This module replaces (supersedes) and extends features defined in [[!CSS2]] section
	<a href="https://www.w3.org/TR/CSS2/visufx.html#overflow-clipping">11.1 Overflow and clipping</a>
	and [[!CSS-UI-3]] section
	<a href="https://www.w3.org/TR/css-ui-3/#text-overflow">5.2. Overflow Ellipsis: the text-overflow property</a>.

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<h2 id="overflow-concepts">
Overflow Concepts and Terminology</h2>

	CSS uses the term <dfn export>overflow</dfn> to describe
	the contents of a box
	that extend outside one of that box's edges
	(i.e., its <i>content edge</i>, <i>padding edge</i>,
	<i>border edge</i>, or <i>margin edge</i>).
	The term might be interpreted as
	elements or features that cause this overflow,
	the non-rectangular region occupied by these features,
	or, more commonly, as
	the minimal rectangle that bounds that region.
	A box's overflow is computed based on the layout and styling of the box itself
	and of all descendants whose <a>containing block chain</a>
	includes the box.

	In most cases, <a>overflow</a>
	can be computed for any box
	from the bounds and properties of that box itself,
	plus the <a>overflow</a>
	of each of its children.
	However, this is not always the case; for example,
	when ''transform-style: preserve-3d'' [[CSS3-TRANSFORMS]] is used on
	some of the children, any of their descendants with
	''transform-style: preserve-3d'' must also be examined.

	There are two different types of overflow,
	which are used for different purposes by the UA:

	<ul>
		<li><a>ink overflow</a>
		<li><a>scrollable overflow</a>
	</ul>

<h3 id="ink">Ink Overflow</h3>

	The <dfn export>ink overflow</dfn> of a box
	is the part of that box and its contents that
	creates a visual effect outside of
	the box's border box.
	Ink overflow is the overflow of painting effects defined to not affect layout
	or otherwise extend the <a>scrollable overflow area</a>,
	such as <a href="https://www.w3.org/TR/css-backgrounds/#box-shadow">box shadows</a>,
	<a href="">border images</a>,
	<a href="https://www.w3.org/TR/css-text-decor-3/">text decoration</a>,
	overhanging glyphs (with negative side bearings,
	or with ascenders/descenders extending outside the em box),
	<a href="https://www.w3.org/TR/css-ui-3/#outline-props">outlines</a>,
	etc.

	Since some effects in CSS (for example, the blurs in
	'text-shadow' [[CSS-TEXT-3]] and 'box-shadow' [[CSS-BACKGROUNDS-3]],
	which are theoretically infinite)
	do not define what visual extent they cover, the extent
	of the <a>ink overflow</a> is undefined.

	The <dfn export lt="ink overflow region | ink overflow area">ink overflow area</dfn> is the non-rectangular area
	occupied by the <a>ink overflow</a>
	of a box and its contents,
	and the <dfn export>ink overflow rectangle</dfn> is
	the minimal rectangle whose axes are aligned to the box's axes
	and that contains the <a>ink overflow area</a>.
	Note that the <a>ink overflow rectangle</a> is a rectangle
	in the box's coordinate system, but might be non-rectangular
	in other coordinate systems due to transforms. [[CSS3-TRANSFORMS]]

	Any overflow of [=replaced=] content is always [=ink overflow=]
	(as opposed to [=scrollable overflow=]).

<h3 id="scrollable">
Scrollable Overflow</h3>

	The <dfn export>scrollable overflow</dfn> of a box is the
	set of things extending outside of that box's padding edge
	for which a scrolling mechanism needs to be provided.

	The <dfn export lt="scrollable overflow region | scrollable overflow area">scrollable overflow area</dfn> is the non-rectangular region
	occupied by the <a>scrollable overflow</a>, and the
	<dfn export>scrollable overflow rectangle</dfn> is
	the minimal rectangle whose axes are aligned to the box's axes
	and that contains the <a>scrollable overflow area</a>.

	The <a>scrollable overflow area</a> is the union of:
	<ul>
		<li>
			The [=scroll container=]’s own [=padding box=].

		<li>
			All [=line boxes=] directly contained by the [=scroll container=].

		<li>
			The border boxes
			of all boxes for which it is the containing block
			and whose border boxes are positioned not wholly
			in the [=negative scrollable overflow region=],
			accounting for transforms by projecting each box onto
			the plane of the element that establishes its <a>3D rendering context</a>.
			[[!CSS3-TRANSFORMS]]

			Issue: Is this description of handling transforms
			sufficiently accurate?

			Border boxes with zero area
			do not affect the [=scrollable overflow area=].

		<li>
			The margin areas of <a>grid item</a> and <a>flex item</a> boxes
			for which the box establishes a containing block.

			The UA may <em>additionally</em> include
			the margin areas of other boxes
			for which the box establishes a containing block;
			however,
			the conditions under which such margin areas are included
			is undefined in this level.
			<span class="issue">This needs further testing and investigation;
			is therefore deferred in this draft.</span>

		<li>
			The <a>scrollable overflow areas</a> of all of the above boxes
			(including zero-area boxes
			and accounting for transforms as described above),
			provided they themselves have ''overflow: visible''
			(i.e. do not themselves trap the overflow)
			and that <a>scrollable overflow</a> is not already clipped
			(e.g. by the 'clip' property or the 'contain' property).

			Issue: should ''overflow: clip'' also clip the <a>scrollable overflow</a>
			or should it remain a pure paint-time operation,
			which would mean that scrollable overflow, while invisible, would still be scrollable.

			Note: The 'mask-*' properties [[!CSS-MASKING-1]]
			do not affect the <a>scrollable overflow area</a>.

			Issue: Need to evaluate compat of honoring or ignoring 'clip' and 'clip-path'.

		<li>
			Additional padding added
			to the [=end=]-side of the [=scrollable overflow rectangle=]
			as necessary to enable a scroll position
			that satisfies the requirements of ''place-content: end'' alignment.

			Note: This padding represents,
			within the [=scrollable overflow rectangle=],
			the box’s own padding
			so that when its content is scrolled to the end,
			there is padding between the end-edge of its in-flow (or floated) content
			and the border edge of the box.
			It typically ends up being exactly the same size as the box's own padding,
			except in a few cases--
			such as when an out-of-flow positioned element,
			or the visible overflow of a descendent,
			has already increased the size of the [=scrollable overflow rectangle=]
			outside the conceptual “content edge” of the [=scroll container=]’s content.

			<figure>
				<img src="images/scroll-align-padding.jpg">
				<figcaption>
					Issue: Replace this image with a proper SVG.
				</figcaption>
			</figure>

	</ul>

	Additionally, due to Web-compatibility constraints
	(caused by authors exploiting legacy bugs to surreptitiously hide content from visual readers but not search engines and/or speech output),
	UAs must clip any content in the [=negative scrollable overflow region=]
	(thereby behaving as if they had no <a>scrollable overflow</a>
	on the wrong side of the [=scroll origin=]).

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	-->

	Note: The <a>scrollable overflow rectangle</a> is always a rectangle
	in the box's own coordinate system, but might be non-rectangular
	in other coordinate systems due to transforms [[CSS3-TRANSFORMS]].
	This means scrollbars can sometimes appear when not actually necessary.

<h3 id="scrolling">
Scrolling Overflow</h3>

	A box’s [=overflow=] can be visible or clipped.
	CSS also allows a box to be <dfn export>scroll container</dfn>
	that allows the user to scroll clipped parts of its <a>scrollable overflow area</a>
	into view.
	The visual viewport of a <a>scroll container</a>
	(through which the <a>scrollable overflow area</a> can be viewed)
	coincides with its padding box,
	and is called the <dfn export>scrollport</dfn>.
	A box’s <dfn export>nearest scrollport</dfn>
	is the [=scrollport=] of its nearest [=scroll container=] ancestor.

	Scrolling operations can be initiated by the user
	(for example, by manipulating a scrollbar, swiping a touchscreen, or using keyboard controls)
	or by script
	(<span class=non-normative>for example, by the {{scrollIntoView()}} or {{focus()}} APIs</span>).
	The initial position of the [=scrollable overflow rectangle=] within the [=scrollport=]
	before any scrolling operations take effect is the <dfn>initial scroll position</dfn>.
	The [=initial scroll position=]
	is typically dependent on the [=scroll container=]’s [=writing mode=],
	and, unless otherwise specified,
	coincides with its [=scroll origin position=].
	However, the 'align-content' and 'justify-content' properties [[!CSS-ALIGN-3]]
	can be used to change the [=initial scroll position=],
	see [[css-align-3#overflow-scroll-position]].

	A <dfn>scroll position</dfn> is a particular alignment
	of the [=scrollable overflow rectangle=]
	within its [=scrollport=].
	It is associated with a <dfn>scroll offset</dfn>
	which is its distance from the [=scroll origin=].

	The <dfn>scroll origin</dfn> is the anchor coordinate of the [=scrollable overflow rectangle=],
	from which the [=scrollable overflow rectangle=] expands.
	Unless otherwise specified,
	it is the [=block-start=] [=inline-start=] corner of the [=scrollable overflow rectangle=].
	(For example, in a [=flex container=] it is the [=main-start=] [=cross-start=] corner.)
	The area beyond the [=scroll origin=] in either axis
	is considered the <dfn>negative scrollable overflow region</dfn>:
	content rendered here is not accessible to the reader,
	see [[#scrollable]].
	A [=scroll container=] is said to be scrolled to its [=scroll origin=]
	when its [=scroll origin=] coincides with the corresponding corner of its [=scrollport=].
	This [=scroll position=], the <dfn>scroll origin position</dfn>,
	usually, but not always,
	coincides with the [=initial scroll position=].

	<div class="example">
		For example,
		the 'align-content' and 'justify-content' properties [[CSS-ALIGN-3]]
		and [=scroll snapping=] [[CSS-SCROLL-SNAP-1]]
		can change the [=initial scroll position=]
		away from the [=scroll origin position=].
	</div>

	ISSUE: Check whether things like [=baseline alignment=]
	depend on the [=initial scroll position=] or the [=scroll origin position=].

	ISSUE: This doesn't define a coordinate system for [=scroll offsets=].
	Whether they increase downward/rightward,
	block/inline-axis endward,
	or away from the [=scroll origin=] is not defined.
	Should each API define its coordinate model?

	The root viewport, which scrolls the page [=canvas=],
	uses the <a>principal writing mode</a> for determining
	its [=scroll origin=] and [=initial scroll position=].

	Note: In the case where a [=scroll container=] (or one of its ancestors)
	is the target of a graphical transform,
	the UA might need to take this transform into account
	when mapping user inputs to scrolling operations.
	For instance, on a touch screen where the user scrolls
	by directly dragging the content,
	the transform would be expected to be taken into account
	to match the direction of scrolling to the gesture.
	On the other hand, other user inputs
	(such as the Page Down key, or a 1D scroll wheel)
	might be more naturally interpreted ignoring the transform.
	Choosing the appropriate behavior for each scrolling mechanism
	is the responsibility of the UA.

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<h2 id="overflow-properties">
Scrolling and Clipping Overflow</h2>

<h3 id="overflow-control">
Managing Overflow: the 'overflow-x', 'overflow-y', and 'overflow' properties</h3>

	These properties specify whether a box’s [=overflow=] is clipped,
	and if so,
	whether it is a [=scroll container=].

	<pre class=propdef>
		Name: overflow-x, overflow-y, overflow-block, overflow-inline
		Value: visible | hidden | clip | scroll | auto
		Initial: ''visible''
		Applies to: block containers [[!CSS2]], flex containers [[!CSS3-FLEXBOX]], grid containers [[!CSS3-GRID-LAYOUT]], and [=replaced=] elements
		Inherited: no
		Percentages: N/A
		Computed value: usually specified value, but see text
		Animation type: discrete
	</pre>

	The 'overflow-x' property specifies
	the handling of <a>overflow</a> in the horizontal axis
	(i.e., overflow from the left and right sides of the box),
	and the 'overflow-y' property specifies the handling
	of <a>overflow</a> in the vertical axis
	(i.e., overflow from the top and bottom sides of the box).

	The 'overflow-block' and 'overflow-inline' properties likewise
	specify the handling of [=overflow=]
	in the [=block axis|block=] and [=inline axis|inline=] axis, respectively

	These four properties form a [=logical property group=]
	together with the 'overflow' [=shorthand=],
	and interact as defined in [[css-logical-1#box]].

	<pre class="propdef shorthand">
		Name: overflow
		Value: [ visible | hidden | clip | scroll | auto ]{1,2}
		Initial: visible
		Applies to: block containers [[!CSS2]], flex containers [[!CSS3-FLEXBOX]], and grid containers [[!CSS3-GRID-LAYOUT]]
		Inherited: no
		Percentages: N/A
		Computed value: see individual properties
		Animation type: discrete
	</pre>

	The 'overflow' property is a [=shorthand property=]
	that sets the specified values of 'overflow-x' and 'overflow-y' in that order.
	If the second value is omitted, it is copied from the first.

	Values have the following meanings:

	<dl dfn-for="overflow, overflow-x, overflow-y" dfn-type="value">
		<dt><dfn>visible</dfn>
		<dd>
			There is no special handling of overflow, that is,
			the box’s content is rendered outside the box if positioned there.
			The box is not a <a>scroll container</a>.

		<dt><dfn>hidden</dfn>
		<dd>
			This value indicates that
			the box’s content is clipped to its [=padding box=]
			and that the UA must not provide any scrolling user interface
			to view the content outside the clipping region,
			nor allow scrolling by direct intervention of the user,
			such as dragging on a touch screen
			or using the scrolling wheel on a mouse.
			However, the content must still be scrollable programmatically,
			for example using the mechanisms defined in [[CSSOM-VIEW]],
			and the box is therefore still a <a>scroll container</a>.

		<dt><dfn>clip</dfn>
		<dd>
			This value indicates that
			the box’s content is clipped to its [=overflow clip edge=]
			and that no scrolling user interface should be provided by the UA
			to view the content outside the clipping region.
			In addition, unlike ''overflow: hidden''
			which still allows programmatic scrolling,
			''overflow: clip'' forbids scrolling entirely,
			through any mechanism,
			and therefore the box is not a <a>scroll container</a>.

			Unlike ''hidden'', this value <strong>does not</strong> cause
			the element to establish a new formatting context.

			Note: Authors who also want the box to establish a formatting context
			may use ''display: flow-root'' together with ''overflow: clip''.

		<dt><dfn>scroll</dfn>
		<dd>
			This value indicates that
			the content is clipped to the [=padding box=],
			but can be scrolled into view
			(and therefore the box is a <a>scroll container</a>).
			Furthermore, if the user agent uses a scrolling mechanism
			that is visible on the screen (such as a scroll bar or a panner),
			that mechanism should be displayed
			whether or not any of its content is clipped.
			This avoids any problem with scrollbars appearing
			and disappearing in a dynamic environment.
			When the target medium is ''@media/print'',
			overflowing content may be printed;
			it is not defined where it may be printed.

		<dt><dfn>auto</dfn>
		<dd>
			Like ''overflow/scroll'' when the box has <a>scrollable overflow</a>;
			like ''overflow/hidden'' otherwise.
			Thus, if the user agent uses a scrolling mechanism
			that is visible on the screen (such as a scroll bar or a panner),
			that mechanism will only be displayed
			if there is overflow.
		</dl>

	The  ''visible''/''overflow/clip'' values of 'overflow'
	compute to ''overflow/auto''/''hidden'' (respectively)
	if one of 'overflow-x' or 'overflow-y' is neither ''visible'' nor ''overflow/clip''.

	If the computed value of 'overflow' on a <a>block box</a>
	is neither ''overflow/visible'' nor ''overflow/clip'' nor a combination thereof,
	it [=establishes an independent formatting context=] for its contents.

<h4 id="scroll-visibility">
Interaction of 'visibility' and 'overflow'</h4>

	If the computed value of the 'visibility' property is ''visibility/hidden''
	(or ''visibility/collapse'' when it has the same effect as ''visibility/hidden''),
	and 'overflow' is either ''overflow/scroll'' or ''overflow/auto'',
	then:

	* The user agent must not make any scrolling mechanism visible.
		To the extent that the scrolling mechanism that would normally be visible
		in the absence of ''visibility: hidden'' affects layout,
		it continues to do so, but is not painted.
	* As would be the case with ''overflow: hidden'',
		scrolling directly triggered by user interactions is disabled,
		but programmatic scrolling continues to take effect.
	* The lack of interactive direct scrolling is enforced
		even if the user interacts (e.g. with a mouse scrolling wheel)
		with a descendent of the ''visibility: hidden'' [=scroll container=]
		that is itself set to ''visibility: visible''.

<h4 id="static-media">
Overflow in Print and Other Static Media</h4>

	<p class="advisement">
		Since scrolling is not possible in static media
		(such as print)
		authors should be careful to make content accessible in such media,
		for example by using ''@media print, (update: none) { &hellip; }''
		to adjust layout such that all relevant content
		is simultaneously visible.

	On <a>scroll containers</a> in non-interactive media
	with an 'overflow' value of ''overflow/auto'' or ''overflow/scroll''
	(but not ''overflow/hidden'')
	UAs may display an indication of any scrollable overflow,
	such as by displaying scrollbars
	or an ellipsis.

	Note: Not all [=paged media=] is non-interactive:
	for example, e-book readers paginate content,
	but are interactive.

<h3 id="overflow-clip-margin">
Expanding Clipping Bounds: the 'overflow-clip-margin' property</h3>

	<pre class=propdef>
	Name: overflow-clip-margin
	Value: <<visual-box>> || <<length [0,∞]>>
	Initial: ''0px''
	Inherited: no
	Applies to: boxes to which 'overflow' applies
	Computed value: the computed <<length>> and a <<visual-box>> keyword
	Animation type: per computed value if the <<visual-box>> values match; otherwise discrete
	</pre>

	This property defines the <dfn export>overflow clip edge</dfn> of the box,
	i.e. precisely <em>how far</em> outside its bounds
	the box’s content is allowed to paint
	before being clipped
	by effects (such as ''overflow: clip'', above)
	that are defined to clip to the box’s [=overflow clip edge=].

	Values are defined as follows:

	<dl dfn-type=value dfn-for=overflow-clip-margin>
		: <dfn><<visual-box>></dfn>
		::
			Specifies the box edge to use as the [=overflow clip edge=] origin,
			i.e. when the specified offset is zero.

			If omitted,
			defaults to ''overflow-clip-margin/padding-box''.

		: <dfn><<length [0,∞]>></dfn>
		::
			The specified offset dictates
			how much the [=overflow clip edge=] is expanded from
			the specified box edge
			Negative values are invalid.
			Defaults to zero if omitted.
	</dl>

	The [=overflow clip edge=] is shaped in the corners
	exactly the same way as an <a spec=css-backgrounds-3>outer box-shadow</a>
	with a spread radius of the same cumulative offset
	from the box’s [=border edge=].
	See [[css-backgrounds-3#corner-shaping]]
	and [[css-backgrounds-3#shadow-shape]],
	noting in particular the formula for outsets beyond the [=border edge=].

	Note: This property has no effect on boxes
	with ''overflow: hidden'' or ''overflow: scroll'',
	which are not defined to use the [=overflow clip edge=].

<h3 id="overflow-propagation">
Overflow Viewport Propagation</h3>

	UAs must apply the 'overflow-*' values
	set on the root element to the <a>viewport</a>
	when the root element’s 'display' value is not ''display/none''.
	However,
	when the root element is an [[!HTML]] <{html}> element
	(including <a href="https://html.spec.whatwg.org/multipage/introduction.html#html-vs-xhtml">XML syntax for HTML</a>)
	whose 'overflow' value is ''overflow/visible'' (in both axes),
	and that element has as a child
	a <{body}> element whose 'display' value is also not ''display/none'',
	user agents must instead apply the 'overflow-*' values
	of the first such child element to the viewport.
	The element from which the value is propagated must then have
	a used 'overflow' value of ''overflow/visible''.

	Note: Using [=containment=] on the HTML <{html}> or <{body}> elements disables
	this special handling of the HTML <{body}> element.
	See the [[CSS-CONTAIN-1#contain-property]] for details.

	Note: ''overflow: hidden'' on the root element
	might not clip everything outside the [=Initial Containing Block=]
	if the ICB is smaller than the viewport,
	which can happen on mobile.

	If ''overflow/visible'' is applied to the viewport,
	it must be interpreted as ''overflow/auto''.
	If ''overflow/clip'' is applied to the viewport,
	it must be interpreted as ''overflow/hidden''.


<h3 id=smooth-scrolling caniuse=css-scroll-behavior>
Smooth Scrolling: the 'scroll-behavior' Property</h3>

	<pre class=propdef>
	Name: scroll-behavior
	Value: auto | smooth
	Initial: auto
	Applies to: [=scroll containers=]
	Inherited: no
	Computed value:    specified value
	Animatable: no
	Canonical Order: per grammar
	</pre>

	The 'scroll-behavior' property specifies the scrolling behavior for a [=scroll container=],
	when scrolling happens due to navigation,
	scrolling APIs [[!CSSOM-VIEW]],
	or scroll snapping operations not initiated by the user [[!CSS-SCROLL-SNAP-1]].
	Any other scrolls, e.g. those that are performed by the user, are not affected by this property.
	When this property is specified on the root element, it applies to the <a>viewport</a> instead.

	Note: The 'scroll-behavior' property of the HTML <{body}> element is <em>not</em> propagated to the viewport.

	<dl dfn-type=value dfn-for=scroll-behavior>
		<dt><dfn>auto</dfn>
		<dd>
			The [=scroll container=] is scrolled in an <a spec=cssom-view lt="instant scroll">instant</a> fashion.

		<dt><dfn>smooth</dfn>
		<dd>
			The [=scroll container=] is scrolled in a <a spec=cssom-view lt="smooth scroll">smooth</a> fashion
			using a user-agent-defined timing function over a user-agent-defined period of time.
			User agents should follow platform conventions, if any. <!--fingerprint-->
	</dl>

	User agents may ignore this property. <!--fingerprint-->

<h2 id="scrollbar-layout">
Scrollbars and Layout</h2>

<h3 id="scrollbar-sizing">
Scrollbar Contributions to Sizing</h3>

	When reserving space for a scrollar placed at the edge of an element's box,
	the reserved space is inserted between the inner border edge
	and the outer padding edge.
	For the purpose of the <a spec=css-backgrounds>background positioning area</a> and <a spec=css-backgrounds>background painting area</a> however,
	this reserved space is considered to be part of the [=padding box=].

	<div class="example">
		In the following document fragment,
		both an absolutely-positioned element
		and a background image
		are positioned to the top right of the box.

		<xmp highlight=html>
			<style>
			  article {
			    background: top right no-repeat url(circle.png);
			    position: relative;
			    overflow: auto; }
			  aside { position: absolute; top: 0; right: 0; }
			</style>
			<article>
			  <aside>×</aside>
			</article>
		</xmp>

		If no scrollbars are present on <code>&lt;article></code>,
		they will both coincide in the top right padding edge corner.
		However, if scrollbars are present
		then <code>&lt;aside></code> will be completely visible,
		on the right padding-box edge adjacent to the scrollbars;
		whereas the background image will be tucked underneath the scrollbars,
		in the top right corner of the scrollbar-extended <a spec=css-backgrounds>background positioning area</a>.
	</div>

	When the box is intrinsically sized,
	this reserved space is added to the size of its contents.
	It is otherwise subtracted from space alotted to the [=content area=].
	To the extent that the presence of scrollbars can affect sizing,
	UAs must start with the assumption that no scrollbars are needed,
	and recalculate sizes if it turns out they are.

	<div class="example">
		In the following document fragment,
		the outer <code>&lt;article></code> has ''height: auto'', but ''max-height: 5em''.
		The inner <code>&lt;section></code> has large margins and would normally just fit:

		<xmp highlight=html>
			...
			    article { overflow: auto; max-height: 5em;  width: max-content; }
			    section { margin: 2em; line-height: 1 }
			...
			<article>
			  <section>
			    This section has big margins.
			  </section>
			</article>
		</xmp>

		If we assumed that <code>&lt;article></code> needed scrollbars,
		then the height of <code>&lt;section></code>,
		including the single line of text and twice 2em of margins,
		adds up to 5em plus a scrollbar.
		Since that is greater than 5em, the maximum allowed height,
		it seems we made the right assumption and d1 indeed needs scrollbars.

		However, we should have started by assuming that no scrollbars are needed.
		In that case the content height of <code>&lt;article></code>
		is exactly the maximum height of 5em,
		proving that the assumption was correct
		and <code>&lt;article></code> indeed should not have scrollbars.
	</div>

<h3 id="scrollbar-gutter-property">
Reserving space for the scrollbar: the 'scrollbar-gutter' property</h3>

	The space between the inner border edge and the outer padding edge
	which user agents may reserve to display the scrollbar
	is called the <dfn export>scrollbar gutter</dfn>.

	The 'scrollbar-gutter' property gives control to the author
	over the presence of <a>scrollbar gutters</a>
	separately from the ability to control the presence of scrollbars
	provided by the 'overflow' property.

	<pre class="propdef">
	Name: scrollbar-gutter
	Value: auto | stable && both-edges?
	Initial: auto
	Applies to: [=scroll containers=]
	Inherited: no
	Computed value: specified keyword(s)
	Animation type: discrete
	</pre>

	This property affects the presence of <a>scrollbar gutters</a>
	placed at the <a>inline start</a> edge or <a>inline end</a> edge of the box.

	The presence of a <a>scrollbar gutter</a>
	at the <a>block start</a> edge and <a>block end</a> edge of the box
	cannot be controlled in this level,
	and is determined the same way as the presence of <a>scrollbar gutters</a>
	placed at the <a>inline start</a> edge or <a>inline end</a> edge of the box
	when 'scrollbar-gutter' is ''scrollbar-gutter/auto''.

	Scrollbars which by default are placed over the content box
	and do not cause <a>scrollbar gutters</a> to be created
	are called <dfn export>overlay scrollbars</dfn>.
	Such scrollbars are usually partially transparent, revealing the content behind them if any.
	Their appearance and size may vary
	based on whether and how the user is interacting with them.

	Scrollbars which are always placed in a <a>scrollbar gutter</a>,
	consuming space when present,
	are called <dfn export>classic scrollbars</dfn>.
	Such scrollbars are usually opaque.

	Whether <a>classic scrollbars</a> or <a>overlay scrollbars</a> are used,
	the appearance and size of the scrollbar,
	and whether scrollbars appear on the start or end edge of the box,
	is UA defined.

	Note: Which side a scrollbar appears on may depend on
	operating system conventions,
	[=bidirectionality=],
	or other ergonomic considerations.

	In the case of <a>classic scrollbars</a>,
	the width of the <a>scrollbar gutter</a>,
	if present (see below),
	is the same as the width of the scrollbar.
	In the case of <a>overlay scrollbars</a>,
	no <a>scrollbar gutter</a> is present.

	Note: There are known use cases that could be addressed
	by enabling <a>scrollbar gutters</a>
	for <a>overlay scrollbars</a>,
	but no satisfactory design has been agreed to so far.
	This could be addressed by future extensions of this property.
	See [[css-overflow-4#sbg-ext]].

	The values of this property have the following meaning:

	<dl dfn-for="scrollbar-gutter" dfn-type=value>
		<dt><dfn>auto</dfn>
		<dd><a>Classic scrollbars</a> consume space by creating a <a>scrollbar gutter</a>
		when 'overflow' is ''overflow/scroll'',
		or when 'overflow' is ''overflow/auto'' and the box is overflowing.
		<a>Overlay scrollbars</a> do not consume space.

		<dt><dfn>stable</dfn>
		<dd>The <a>scrollbar gutter</a> is present
		for <a>classic scrollbars</a>
		when 'overflow' is ''overflow/hidden'', ''overflow/scroll'', or ''overflow/auto'',
		regardless of whether the box is actually overflowing.
		<a>Overlay scrollbars</a> do not consume space.

		Note: This does not change whether the scrollbar itself is visible,
		only the presence of a gutter is affected.

		<dt><dfn>both-edges</dfn>
		<dd>If a <a>scrollbar gutter</a> would be present
		on one of the inline start edge or the inline end edge of the box,
		another <a>scrollbar gutter</a> must be present on the opposite edge as well.
	</dl>

	When the <a>scrollbar gutter</a> is present but the scrollbar is not,
	or the scrollbar is transparent or otherwise does not fully obscure the <a>scrollbar gutter</a>,
	the background of the <a>scrollbar gutter</a> must be painted as an extension of the padding.

	As for the 'overflow' property,
	when 'scrollbar-gutter' is set on the root element,
	the user agent must apply it to the viewport instead,
	and the used value on the root element itself is ''scrollbar-gutter: auto''.
	However, unlike the 'overflow' property,
	the user agent must not propagate 'scrollbar-gutter' from the HTML <{body}> element.

	<div class=note>
	Note: The following table summarizes the interaction of 'overflow' and 'scrollbar-gutter'
	showing in which case space is reserved for a <a>classic scrollbar</a>'s <a>scrollbar gutter</a>.

	<style>
	.data.complex tbody th:last-of-type {
		border-right: solid black 2px;
	}
	.data.complex tbody tr:nth-of-type(2n) td,
	.data.complex tbody tr:nth-of-type(2n) th:not([rowspan]) {
		background: rgba(127, 127, 127, 0.25);
	}
	</style>

	<table class="data complex">
		<caption>
			Should a <a>classic scrollbar</a>'s <a>scrollbar gutter</a> be present?</caption>
		<thead>
		<tr>
			<th>'overflow'
			<th>'scrollbar-gutter'
			<th>Overflowing
			<th>Not overflowing
		</thead>
		<tr>
			<th rowspan=2>''overflow/scroll''
			<th>''scrollbar-gutter/auto''
			<td>yes
			<td>yes
		<tr>
			<th>''scrollbar-gutter/stable''
			<td>yes
			<td>yes
		<tr>
			<th rowspan=2>''overflow/auto''
			<th>''scrollbar-gutter/auto''
			<td>yes
			<td>
		<tr>
			<th>''scrollbar-gutter/stable''
			<td>yes
			<td>yes
		<tr>
			<th rowspan=2>''overflow/hidden''
			<th>''scrollbar-gutter/auto''
			<td>
			<td>
		<tr>
			<th>''scrollbar-gutter/stable''
			<td>yes
			<td>yes
		<tr>
			<th rowspan=2>''overflow/visible'', ''overflow/clip''
			<th>''scrollbar-gutter/auto''
			<td>
			<td>
		<tr>
			<th>''scrollbar-gutter/stable''
			<td>
			<td>
	</table>
	</div>

<h2 id="auto-ellipsis">
Automatic Ellipses</h2>

<h3 id="text-overflow" caniuse="text-overflow">
Overflow Ellipsis: the 'text-overflow' property</h3>

	<pre class="propdef">
	Name: text-overflow
	Value: clip | ellipsis
	Initial: clip
	Applies to: block containers
	Inherited: no
	Percentages: N/A
	Computed value: specified keyword
	Animation type: discrete
	</pre>

	This property specifies rendering when inline content overflows
	its <a>end</a> line box edge
	in the inline progression direction of its block container element ("the block")
	that has 'overflow'
	other than ''visible''.

	Text can overflow for example when it is prevented from wrapping
	(e.g. due to <code class="lang-css">white-space: nowrap</code>
	or a single word is too long to fit).
	Values have the following meanings:

	<dl data-dfn-for="text-overflow" class="valuedef">
		<dt><dfn id=overflow-clip dfn-type=value>clip</dfn>
		<dd>
			Clip inline content that overflows its block container element. Characters may be only partially rendered.

		<dt><dfn id=overflow-ellipsis dfn-type=value>ellipsis</dfn>
		<dd>
			Render an ellipsis character (U+2026)
			to represent clipped inline content.
			Implementations may substitute a more language, script, or writing-mode appropriate
			ellipsis character,
			or three dots "..." if the ellipsis character is unavailable.
	</dl>

	The term "character" is used in this property definition
	for better readability and means "grapheme cluster" [[!UAX29]]
	for implementation purposes.

	For the ellipsis value
	implementations must hide characters and
	<a href="https://www.w3.org/TR/CSS2/visuren.html#inline-boxes">
	atomic inline-level elements</a>
	at the <a>end</a> edge of the line as necessary to fit the ellipsis, and
	place the ellipsis immediately adjacent
	to the <a>end</a> edge of the remaining inline content.
	The first character or
	<a href="https://www.w3.org/TR/CSS2/visuren.html#inline-boxes">
	atomic inline-level element</a>
	on a line
	must be clipped rather than ellipsed.

	<div class=example>
		<h4 id="bidi-ellipsis" class="no-num no-toc">Bidi ellipsis examples</h4>

		These examples demonstrate which characters get hidden
		to make room for the ellipsis in a bidi situation:
		those visually at the end edge of the line.

		Sample CSS:
		<pre highlight=css>
		div {
		  font-family: monospace;
		  white-space: pre;
		  overflow: hidden;
		  width: 9ch;
		  text-overflow: ellipsis;
		}
		</pre>

		Sample HTML fragments, renderings, and your browser:
		<table class="awesome-table data">
		<thead><th>HTML<th>Reference rendering<th>Your Browser</thead>
		<tr>
			<td><code highlight=html>&lt;div>שלום 123456&lt;/div></code><td><div style="font-family:monospace">123456 ם…</div><td><div style="font-family: monospace; white-space: pre; overflow: hidden; width: 9ch; text-overflow: ellipsis">שלום 123456</div>
			<tr><td><code highlight=html>&lt;div dir=rtl>שלום 123456&lt;/div></code><td><div style="font-family:monospace">…456 שלום</div><td><div style="font-family: monospace; white-space: pre; overflow: hidden; width: 9ch; text-overflow: ellipsis" dir=rtl>שלום 123456</div>
		</table>
	</div>

<h4 id="ellipsing-details" class="no-num no-toc">
ellipsing details</h4>

	<ul>
		<li>
			Ellipsing only affects rendering and must not affect layout
			nor dispatching of pointer events:
			The UA should dispatch any pointer event on the ellipsis to the elided element,
			as if 'text-overflow' had been ''text-overflow/none''.

		<li>
			The ellipsis is styled and baseline-aligned according to
			the block.

		<li>
			Ellipsing occurs after relative positioning and other graphical transformations.

		<li>
			If there is insufficient space for the ellipsis,
			then clip the rendering of the ellipsis itself
			(on the same side that neutral characters on the line
			would have otherwise been clipped with the ''text-overflow:clip'' value).
	</ul>

<h4 id="ellipsis-interaction" class="no-num no-toc">
user interaction with ellipsis</h4>

	<ul>
		<li>
			When the user is interacting with content
			(e.g. editing, selecting, scrolling),
			the user agent may treat ''text-overflow: ellipsis'' as ''text-overflow: clip''.

		<li>Selecting the ellipsis should select the ellipsed text.
			If all of the ellipsed text is selected,
			UAs should show selection of the ellipsis.
			Behavior of partially-selected ellipsed text is up to the UA.
	</ul>

	<div class="example"><p style="display:none">Example(s):

		<h4 id="text-overflow-examples" class="no-num no-toc">text-overflow examples</h4>

		These examples demonstrate setting the text-overflow of a block container element
		that has text which overflows its dimensions:

		sample CSS for a div:
		<pre highlight=css>
		div {
			font-family:Helvetica,sans-serif; line-height:1.1;
			width:3.1em; padding:.2em; border:solid .1em black; margin:1em 0;
		}
		</pre>

		sample HTML fragments, renderings, and your browser:

		<table class="awesome-table">
			<tr>
				<th>HTML
				<th>sample rendering
				<th>your browser

			<tr>
				<td>
					<pre highlight=html>
					&lt;div&gt;
					CSS IS AWESOME, YES
					&lt;/div&gt;
					</pre>

				<td>
					<object type="image/png" data="images/cssisawesome.png">
						First, a box with text drawing outside of it.
					</object>

				<td>
					<div style="width:3.1em; border:solid .1em black; margin:1em 0; padding:.2em; font-family:Helvetica,sans-serif; line-height:1.1;">CSS IS AWESOME, YES</div>

			<tr>
				<td>
					<pre highlight=html>
					&lt;div style="<strong>text-overflow:clip;</strong> overflow:hidden"&gt;
					CSS IS AWESOME, YES
					&lt;/div&gt;
					</pre>

				<td>
					<object type="image/png" data="images/cssisaweso.png">
						Second, a similar box with the text clipped outside the box.
					</object>

					<td>
						<div style="width:3.1em; border:solid .1em black; margin:1em 0; padding:.2em; font-family:Helvetica,sans-serif; line-height:1.1; overflow:hidden;text-overflow:clip;">CSS IS AWESOME, YES</div>

			<tr>
				<td>
					<pre highlight=html>
					&lt;div style="<strong>text-overflow:ellipsis;</strong> overflow:hidden"&gt;
					CSS IS AWESOME, YES
					&lt;/div&gt;
					</pre>

				<td>
					<object type="image/png" data="images/cssisaw.png">
						Third, a similar box with an ellipsis representing the clipped text.
					</object>

				<td>
					<div style="width:3.1em; border:solid .1em black; margin:1em 0; padding:.2em;  font-family:Helvetica,sans-serif; line-height:1.1; overflow:hidden;text-overflow:ellipsis;">CSS IS AWESOME, YES</div>

			<tr>
				<td>
					<pre highlight=html>
					&lt;div style="<strong>text-overflow:ellipsis;</strong> overflow:hidden"&gt;
					NESTED
					 &lt;p&gt;PARAGRAPH&lt;/p&gt;
					WON'T ELLIPSE.
					&lt;/div&gt;
					</pre>

				<td>
					<object type="image/png" data="images/nes.png">
						Fourth, a box with a nested paragraph demonstrating anonymous block boxes equivalency and non-inheritance into a nested element.
					</object>

				<td>
					<div style="width:3.1em; border:solid .1em black; margin:1em 0; padding:.2em;  font-family:Helvetica,sans-serif; line-height:1.1; overflow:hidden;text-overflow:ellipsis;">NESTED
						<p>PARAGRAPH</p>
						WON'T ELLIPSE.</div>

			</table>
	</div>

	Note: the side of the line that the ellipsis is placed depends on the 'direction' of the block.
	E.g. an overflow hidden right-to-left
	(<code class="lang-css">direction: rtl</code>)
	block clips inline content on the <a spec=css-writing-modes-3>left</a> side,
	thus would place a text-overflow ellipsis on the <a spec=css-writing-modes-3>left</a>
	to represent that clipped content.

	<!-- insert RTL example diagram here to illustrate note. -->

<h4 id="ellipsis-scrolling" class="no-num no-toc">
ellipsis interaction with scrolling interfaces</h4>

	This section applies to elements with text-overflow other than ''text-overflow:clip''
	(non-clip text-overflow)
	and overflow:scroll.

	When an element with non-clip text-overflow has overflow of scroll
	in the inline progression dimension of the text,
	and the browser provides a mechanism for scrolling
	(e.g. a scrollbar on the element,
	or a touch interface to swipe-scroll, etc.),
	there are additional implementation details that provide a better user experience:

	When an element is scrolled (e.g. by the user, DOM manipulation),
	more of the element's content is shown.
	The value of text-overflow should not affect
	whether more of the element's content is shown or not.
	If a non-clip text-overflow is set,
	then as more content is scrolled into view,
	implementations should show whatever additional content fits,
	only truncating content which would otherwise be clipped
	(or is necessary to make room for the ellipsis/string),
	until the element is scrolled far enough
	to display the edge of the content
	at which point that content should be displayed
	rather than an ellipsis/string.

	<div class="example">

		This example uses text-overflow on an element with overflow scroll
		to demonstrate the above described behavior.

		sample CSS:

		<pre highlight=css>
		div.crawlbar {
			text-overflow: ellipsis;
			height: 2em;
			overflow: scroll;
			white-space: nowrap;
			width: 15em;
			border:1em solid black;
		}
		</pre>

		sample HTML fragment:

		<pre highlight=html>
		&lt;div class="crawlbar"&gt;
		CSS is awesome, especially when you can scroll
		to see extra text instead of just
		having it overlap other text by default.
		&lt;/div&gt;
		</pre>

		demonstration of sample CSS and HTML:

		<div style="text-overflow: ellipsis; height: 2em; overflow: scroll; white-space: nowrap; width: 15em; border:1em solid black;">
		CSS is awesome, especially when you can scroll
		to see extra text instead of just
		having it overlap other text by default.
		</div>
	</div>

	While the content is being scrolled, implementations may adjust their rendering of ellipses (e.g. align to the box edge rather than line edge).

<h3 id="block-ellipsis">
Indicating Block-Axis Overflow: the 'block-ellipsis' property</h3>

	<pre class=propdef>
		Name: block-ellipsis
		Value: none | auto | <<string>>
		Initial: none
		Applies to: [=block containers=]
		Inherited: yes
		Percentages: N/A
		Computed value: specified value
		Animation type: discrete
	</pre>

	This property allows inserting content into the last line box
	before a (forced <em>or</em> unforced) <a>region break</a>
	to indicate the continuity of truncated/interrupted content.
	It only affects line boxes contained directly by the <a>block container</a> itself,
	but as it inherits, will have an effect on descendants’ line boxes unless overridden.
	If the box contains no line box immediately preceding a <a>region break</a>,
	then this property has no effect.

	Note: See [[#fragmentation]] for a way to generate boxes with such a [=region break=].

	ISSUE: Should this apply to other types of fragmentation breaks (e.g. pages, columns)?

	The inserted content is called the <dfn>block overflow ellipsis</dfn>.
	Values have the following meanings:

	<dl dfn-for="block-ellipsis" dfn-type=value>
		<dt><dfn>none</dfn>
		<dd>
			The rendering is unaffected.

		<dt><dfn>auto</dfn>
		<dd>
			Render an ellipsis character (U+2026)--
			or a more typographically-appropriate equivalent--
			as the <a>block overflow ellipsis</a>
			at the end of the affected line box.
			UAs should use the conventions of the
			<a>content language</a>,
			writing system, and
			<a>writing mode</a>
			to determine the most appropriate ellipsis string.

		<dt><dfn><<string>></dfn>
		<dd>
			Render the specified string
			as the <a>block overflow ellipsis</a>
			at the end of the affected line box.
			The UA may truncate this string if it is absurdly long.
	</dl>

	When 'block-ellipsis' is not ''block-ellipsis/none'',
	the <a>block overflow ellipsis</a> string
	is wrapped in an anonymous inline
	and placed at the end of the line box
	as a direct child of the <a>block container</a>’s <a>root inline box</a>,
	reducing the space in the line box
	available to the other contents of the line.
	This inline is assigned ''unicode-bidi: plaintext''
	and ''line-height: 0''
	and is placed in the line box after the last
	<a>soft wrap opportunity</a> (see [[!CSS-TEXT-3]])
	that would still allow the entire <a>block overflow ellipsis</a> to fit on the line.
	For this purpose, <a>soft wrap opportunities</a> added by 'overflow-wrap' are ignored.
	If this results in the entire contents of the line box being displaced,
	the line box is considered to contain a [=strut=], as defined in [[CSS2/visudet#leading]].
	Text <a href="https://www.w3.org/TR/css-text-3/#justification">alignment and justification</a>
	occurs after placement,
	and measures the inserted <a>block overflow ellipsis</a>
	together with the rest of the line’s content.

	Note: Setting the [=block overflow ellipsis=]'s 'line-height' to ''0''
	makes sure that inserting it cannot cause the line's height to grow,
	which could cause further relayouts and potentially cycles.
	This is almost equivalent to inserting the [=block overflow ellipsis=]
	as a paint-time operation, except that it still participates in alignment and justification.
	The downside is that unusually tall / deep glyphs in the [=block overflow ellipsis=]
	may overflow.

	The [=block overflow ellipsis=] must not be included
	in either the ''::first-letter'' nor the ''::first-line'' pseudo-elements.

	If there is a subsequent <a>fragmentation container</a> in the [=fragmentation context=]
	that would receive subsequent content,
	then the content displaced by the <a>block overflow ellipsis</a>
	must be pushed to that <a>fragmentation container</a>.

	The UA must treat the <a>block overflow ellipsis</a> as an unbreakable string,
	If any part of the [=block overflow ellipsis=] overflows,
	it is treated as [=scrollable overflow=],
	and its rendering is affected by the 'text-overflow' property.

	The <a>block overflow ellipsis</a> does not capture events:
	pointer events are dispatched to whatever is underneath it.

	It also has no effect on the intrinsic size of the box:
	its <a lt="min-content size">min-content</a> and <a lt="max-content size">max-content</a> sizes
	are calculated exactly as if 'block-ellipsis' were ''block-ellipsis/none''.

	Note: Future specifications may extend this feature,
	for example by providing an ''::ellipsis'' pseudo-element
	to style the text,
	or by allowing the selection of a child element of the block
	to use as either an inline-level or block-level indicator
	(in which case, it can capture events).


<h2 id=fragmentation>
Fragmenting Overflow</h2>

<h3 id=line-clamp>
Limiting Visible Lines: the 'line-clamp' shorthand property</h3>

	<pre class="propdef shorthand">
		Name: line-clamp
		Value: none | <<integer>> <<'block-ellipsis'>>?
		Initial: none
		Percentages: N/A
	</pre>

	The 'line-clamp' property is a <a>shorthand</a>
	for the 'max-lines', 'block-ellipsis', and 'continue' properties.

	Issue: For the time being,
	experimental implementations are encouraged
	to follow the full behavior defined by this shorthand and its longhands,
	but to only expose the shorthand to authors.
	This is in order to facilitate further tweaking,
	and in particular potential renaming,
	of the longhand properties and their values.

	It allows limiting the contents of a block container
	to the specified number of lines;
	remaining content is fragmented away
	and neither rendered nor measured.
	Optionally, it also allows inserting content into the last line box
	to indicate the continuity of truncated/interrupted content.

	The values have the following meaning:

	<dl dfn-for=line-clamp dfn-type=value>
		<dt><dfn>none</dfn>
		<dd>Sets 'max-lines' to ''max-lines/none'',
			'continue' to ''continue/auto'',
			and 'block-ellipsis' to ''block-ellipsis/none''.

		<dt><dfn><<integer>></dfn>
		<dd>Sets 'max-lines' to the specified <<integer>>,
			'continue' to ''discard'',
			and the 'block-ellipsis' property to second component of the value
			or to ''block-ellipsis/auto'' if omitted.

	</dl>

	See the corresponding longhand properties for details
	about how this mechanism operates.

	<div class="example">
		In this example, the lead paragraph of each article
		is listed in a shortened menu,
		truncated to fit within 5 lines
		that end with “… (continued on next page)”:

		<xmp highlight=css>
			li {
				line-clamp: 5 "… (continued on next page)";
			}
			strong {
				display: block;
				text-transform: uppercase;
			}
		</xmp>

		<xmp highlight=markup>
			<li><a href="cheese-is-milk">
				<strong>Cheese is Actually Made of Milk!</strong>
				Investigative reporters at the World Wide Web Press Corps
				have discovered the secret of cheese.
				Tracing through byzantine layers of bureaucracy and shadow corporations,
				our crack team of journalists have traced the source of camembert.
			</a></li>
		</xmp>

		Sample rendering:

		<pre class="figure">
			+---------------------------------------+
			| CHEESE IS ACTUALLY MADE OF MILK!      |
			| Investigative reporters at the World  |
			| Wide Web Press Corps have discovered  |
			| the secret of cheese. Tracing through |
			| byzantine…  (continued on next page)  |
			+---------------------------------------+
		</pre>
	</div>

<h4 id=webkit-line-clamp>
Legacy compatibility</h4>

	For compatibility with legacy content,
	UAs that support 'line-clamp' must also support the '-webkit-line-clamp' property
	and the additional ''-webkit-discard'' value for the 'continue' property.

	<pre class="propdef shorthand">
		Name: -webkit-line-clamp
		Value: none | <<integer>>
		Initial: none
		Percentages: N/A
	</pre>

	<pre class="propdef partial">
		Name: continue
		New Values: -webkit-discard
	</pre>

	Like 'line-clamp', '-webkit-line-clamp' is a shorthand of 'max-lines', 'continue', and 'block-ellipsis',
	except that:

	* its syntax is ''line-clamp/none'' | <<integer>>
	* it sets 'continue' to ''-webkit-discard'' instead of ''discard''
	* it unconditionally sets 'block-ellipsis' to ''block-ellipsis/auto''

	Additionally, for children (including anonymous children)
	of boxes whose 'display' property computes to ''-webkit-box'' or ''-webkit-inline-box'',
	the used values of the 'max-lines', 'continue', and 'block-ellipsis' properties
	are taken from the computed values of the parent box;
	the computed values of these properties on the box itself are ignored.

	The <dfn value for=continue>-webkit-discard</dfn> value behaves identically to ''discard'',
	except that it only takes effect
	if the computed value of the 'display' property on the parent
	is ''-webkit-box'' or ''-webkit-inline-box''
	and the computed value of the '-webkit-box-orient' property on the parent
	is ''-webkit-box-orient/vertical''.

	Note: Implementations of the legacy '-webkit-line-clamp' property
	have not behaved identically to what is specified here.
	The historical behavior is quirky and less robust,
	as documented for example in <a href="https://medium.com/mofed/css-line-clamp-the-good-the-bad-and-the-straight-up-broken-865413f16e5">this blog post</a>.
	The current design learns from the mistakes of that early experiment,
	and is intended to be sufficiently compatible with existing content
	that implementations can eventually be changed to follow to the specified behavior.
	If further adjustments are found to be necessary,
	they will be incorporated to this specification.
	In the meanwhile, authors should be aware that there may be discrepancies.

<h3 id="max-lines">
Forcing a Break After a Set Number of Lines: the 'max-lines' property</h3>

	<pre class=propdef>
		Name: max-lines
		Value: ''none'' | <<integer [1,∞]>>
		Initial: ''none''
		Applies to: <a>block containers</a> which are also [=fragmentation containers=] that capture [=region breaks=]
		Inherited: no
		Percentages: N/A
		Computed value: the keyword ''max-lines/none'' or an integer
		Animation type: by computed value type
	</pre>

	This property has no effect on boxes that are not [=fragmentation containers=] that capture [=region breaks=].

	Otherwise, if the value of 'max-lines' is not <dfn for=max-lines dfn-type=value>none</dfn>,
	a <a>region break</a>
	is forced after its <var>N</var>th
	descendant <a>in-flow</a> <a>line box</a>,
	where <var>N</var> is the specified value of 'max-lines'.
	Only lines boxes in the same <a>Block Formatting Context</a>
	are counted:
	the contents of descendants that establish <a>independent formatting contexts</a>
	are skipped over while counting line boxes.

	If fewer than <var>N</var> line boxes exist,
	then 'max-lines' introduces no <a>region break</a>.

	<div class=example>
		As ''continue: discard'' does not cause the element to [=establish an independent formatting context=],
		lines in a nested element with 'line-clamp' are counted,
		as can be seen in the examples below.
		<xmp highlight=markup>
			<div id=a>
				a: line 1<br>
				a: line 2<br>
				<div id=b>
					b: line 1<br>
					b: line 2<br>
					b: line 3<br>
					b: line 4<br>
				</div>
				a: line 3<br>
				a: line 4<br>
			</div>
		</xmp>
		Sample rendering given <code highlight=css>#a { line-clamp: 5; } #b { line-clamp: 2; }</code>:
		<pre>
			a: line 1
			a: line 2
			b: line 1
			b: line 2…
			a: line 3…
		</pre>

		Sample rendering given <code highlight=css>#a { line-clamp: 3; } #b { line-clamp: 2; }</code>:
		<pre>
			a: line 1
			a: line 2
			b: line 1…
		</pre>
		Note that in the second case,
		the maximum of 2 lines set on element ''#b'' does not take effect,
		since a forced break is introduced
		before the second line of this element.
	</div>

	Note: This implies that 'max-lines' has no effect when applied to [=multi-column containers=],
	since any line box they contain are nested into [=independent formatting contexts=].

	Only positive integers are accepted.
	Zero or negative integers are invalid
	and must cause the declaration to be <a>ignored</a>.

	Note: The 'widows', 'orphans', and 'break-inside' properties
	do not affect the position of the forced <a>region break</a>
	introduced by the 'max-lines' property.

	<div class=note>
		Note: Despite the “region break” name, this is not a dependency on [[CSS-REGIONS-1]].
		The word “region” is only used as a classifier for forced breaks:
		they can be “page breaks” (breaks across pages [[css-page-3]]),
		“column breaks” (breaks across multi-column layout columns [[css-multicol-1]]),
		or “region breaks” (breaks across any other kind of CSS-induced <a>fragmentation containers</a>).

		If an implementation supports neither [[CSS-REGIONS-1]] nor [[CSS-OVERFLOW-4#fragmentation]],
		then it will have had no occasion yet to run into that kind of breaks,
		and this will be an addition.
		However the addition does not involve bringing over any of the [[CSS-REGIONS-1]] functionality.
		All that is needed is:

		* be able to fragment
		* classify these fragmentation containers as “Category 3”
			(i.e. not pages nor columns)
			for the purpose of forced breaks.
	</div>

<h3 id="continue">
Fragmentation of Overflow: the 'continue' property</h3>

	<pre class=propdef>
		Name: continue
		Value: auto | discard
		Initial: ''continue/auto''
		Applies to: [=block containers=] and [=multicol containers=]
		Inherited: no
		Percentages: N/A
		Computed value: specified keyword
		Animation type: discrete
	</pre>

	The 'continue' property gives authors the ability
	to turn a box into a [=fragmentation container=] (see [[!CSS-BREAK-3]])
	and to specify that content after the [=fragmentation break=] must be discarded.

	Issue: This property is meant to generalize and replace the <code>region-fragment</code> property from [[CSS-REGIONS-1]].
	Once it is sufficiently stable in this specification,
	<code>region-fragment</code> should be removed from the regions specification in favor of this.

	<dl dfn-for="continue" dfn-type="value">
		<dt><dfn>auto</dfn>
		<dd>

			If the box has more content than can fit,
			the excess content is handled according to the usual rules.

		<dt><dfn>discard</dfn>
		<dd>
			The box becomes a [=fragmentation container=]
			that captures <a>region breaks</a>,
			if it is not already.
			[[!CSS-BREAK-3]]
			Content after the first [=region break=] is not rendered (see below).
			(If the box is a [=multi-column container=],
			any [=overflow columns=] are also not rendered.)


			Note: This [=region break=] might be <a lt="forced break">forced</a>
			(e.g. imposed by 'max-lines' or by another mechanism,
			such as the 'break-before'/'break-after' properties)
			or [=unforced break|unforced=]
			(e.g. if the content would otherwise overflow this [=fragmentation container=]
			due to its size constraints).
			Breaks applying to other [=fragmentation contexts=]
			(such as pagination of this box itself)
			do not cause any content to be discarded.
	</dl>

	Note: This property does <em>not</em> cause the box to [=establish an independent formatting context=].

	<div class=example>
		Given an article with one excessively long overflowing line,
		and four more lines than can fit in the [=fragmentation direction=] as in the illustration bellow,
		different renderings are possible based on the combination of the 'overflow' and 'continue' property.

		<figure>
		<img src="images/continue-flow.png" alt="article with one excessively long line and four more that can fit in the block direction">
		</figure>


		<table class=data>
			<tr>
				<th>
				<th>''continue: discard''
				<th>''continue: auto''

			<tr>
				<th>''overflow: visible''

				<td style=vertical-align:top><img src="images/continue-discard-visible.png" alt=
				"rendering with continue:discard and overflow:visible"></td>

				<td><img src="images/continue-auto-visible.png"
				alt="rendering with continue:auto and overflow:visible"></td>
			</tr>

			<tr>
				<th>''overflow: hidden''

				<td><img src="images/continue-discard-hidden.png" alt=
				"rendering with continue:discard and overflow:hidden"></td>

				<td><img src="images/continue-auto-hidden.png"
				alt="rendering with continue:auto and overflow:hidden"></td>
			</tr>
		</table>

	</figure>
	</div>

	Content that is “not rendered” due to ''continue: discard''
	is discarded, similar to ''display: none'':
		* It is not rendered.
		* It is also not made available for speech rendering.
		* It does not allow user interaction.
		* Issue(2970): Make sure effects on OM are well defined
		* Issue(2971): What about [=positioned=] elements whose static position is in the discarded content are not rendered?
			See also discussions in the <a href="https://lists.w3.org/Archives/Public/www-style/2018Jul/0030.html">Sydney F2F meeting</a>.

	However, since intrinsic sizes are calculated across <a>fragmentation containers</a>,
	this content <em>is</em> taken into account
	for the purpose of finding the box’s
	<a lt="min-content inline size">min-content</a> and <a>max-content</a> <a>inline sizes</a>
	(see [[css-break-3#varying-size-boxes]]).
	<a lt="min-content inline size">Min-content</a> and <a>max-content</a> <a>block sizes</a>
	are calculated based on the content
	from the start of the [=fragmented flow=]
	to the first [=forced break=] if any,
	or to the end of the [=fragmented flow=] if there is no forced break.

<!--
	Calculating intrinsic block sizes to the first forced <em>or unforced</em> break
	would cause intrinsic sizes to depend on layout, which would cause circularities.
-->

	Note: In the case of <a href="https://www.w3.org/TR/css-break-3/#parallel-flows">parallel fragmentation flows</a>,
	content occurring after the <a>fragmentation break</a> in the box tree
	could still be rendered,
	if it is laid out above the position
	representing the end of this <a>fragmentation container</a>.

<h2 class="no-num non-normative" id="priv">Appendix A. Privacy Considerations</h2>

	This specification introduces no new privacy concerns.

<h2 class="no-num non-normative" id="sec">Appendix B. Security Considerations</h2>

	This specification introduces no new security concerns.

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

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

<h3 id="changes-since-2021-12-02">
Changes from the <a href="https://www.w3.org/TR/2021/WD-css-overflow-3-20211202/">2021-12-02 Working Draft</a></h3>


	* Require end-edge [=padding=] to be included
		in the [=scrollable overflow area=] of [=block containers=];
		this was previously optional for [=block containers=] with ''align-content/normal'' [=content distribution=],
		pending Web-compatibility investigations.
		(<a href="https://github.com/w3c/csswg-drafts/issues/129#issuecomment-1113489051">Issue 129</a>)
	* More precisely define the interaction of '-webkit-line-clamp'
		and the various 'line-clamp' longhands. See [[#webkit-line-clamp]].
		(<a href="https://github.com/w3c/csswg-drafts/issues/6842">Issue 6842</a>)
	* Clarified that [=region breaks=] are what trigger discarding of subsequent content
		for ''continue: discard'' (and that other types of breaks do not);
		and gave examples of what triggers a [=region break=] on such a box.
	* Minor clarifications:
		* Annotate <<length-percentage>> value definitions using [=CSS bracketed range notation=].

<h3 id="changes-since-2020-06-03">
Changes from the <a href="https://www.w3.org/TR/2020/WD-css-overflow-3-20200603/">2020-06-03 Working Draft</a></h3>

	* Moved the 'scroll-behavior' property from [[CSSOM-VIEW]] to this specification.
	* Adopted the 'scrollbar-gutter' property, previously defined in [[CSS-OVERFLOW-4]].
	* Added box-edge keywords to 'overflow-clip-margin'.
		(<a href="https://github.com/w3c/csswg-drafts/issues/5801">Issue 5801</a>)
	* Clarified interaction of 'border-radius' and 'overflow-clip-margin'.
	* Added note about user scrolling gestures vs. transforms.
	* Tightened up requirements around incorporating margins, padding, and zero-area boxes into the [=scrollable overflow area=].
		(<a href="https://github.com/w3c/csswg-drafts/issues/129">Issue 129</a>,
		<a href="https://github.com/w3c/csswg-drafts/issues/4791">Issue 4791</a>)
	* More precisely defined impact of scrollbars on layout.
		(<a href="https://github.com/w3c/csswg-drafts/issues/3348">Issue 3348</a>)
	* Handle various edge cases involving 'overflow' propagation to the viewport.
		(<a href="https://github.com/w3c/csswg-drafts/issues/3779">Issue 3779</a>,
		<a href="https://github.com/w3c/csswg-drafts/issues/5913">Issue 5913</a>)
	* Miscellaneous small clarifications and fixes.

<h3 id="changes-since-2018-07-31">
Changes from the <a href="https://www.w3.org/TR/2018/WD-css-overflow-3-20180731/">2018-07-31 Working Draft</a></h3>

	* Fixed markup errors and typos
	* Made various editorial clarifications and fixed inconsistencies
	* Renamed <code>block-overflow</code> to 'block-ellipsis'
	* Define 'text-overflow' to affect an overflowing [=block overflow ellipsis=]
	* Define the behavior of a [=block overflow ellipsis=] that is longer than the line
	* Define interaction of 'block-ellipsis' and ''::first-line'' / ''::first-letter''
	* Adjust the intrinsic size of elements with ''continue: discard''
	* Define how to layout the [=block overflow ellipsis=]
	* Define the legacy behavior of '-webkit-line-clamp' for compatibility with existing content
	* Define that end padding is included in the scrollable overflow area except for block containers
		(which are more tricky to handle, due to web-compat issues)
	* Define that Margins of flex and grid items are included in scrollable overflow
	* Do not propagate overflow from boxless elements
	* Rename 'scrollable overflow region' to 'scrollable overflow area'
	* Introduce the 'overflow-clip-margin' property

<h3 id="old-changes">
Changes Prior to the <a href="https://www.w3.org/TR/2018/WD-css-overflow-3-20180731/">2018-07-31 Working Draft</a></h3>

	Changes predating the publication of the <a href="https://www.w3.org/TR/2018/WD-css-overflow-3-20180731/">2018-07-31 Working Draft</a>
	can be found in the following change logs:

	* <a href="https://github.com/w3c/csswg-drafts/commits/master?path=css-overflow-3&until=2018-07-31">from 2017-08-02 to 2018-07-31</a>
	* <a href="https://hg.csswg.org/drafts/log/tip/css-overflow/Overview.bs">from 2015-01-27 to 2017-08-01</a>
	* <a href="https://hg.csswg.org/drafts/log/tip/css-overflow/Overview.src.html">from 2013-03-28 to 2015-01-27</a>
	* <a href="https://hg.csswg.org/drafts/log/tip/css3-overflow/Overview.src.html">from 2012-07-31 to 2013-03-27</a>

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

	Thanks especially to the feedback from
	Rossen Atanassov,
	Bert Bos,
	Tantek Çelik,
	John Daggett,
	Daniel Glazman,
	Vincent Hardy,
	H&aring;kon Wium Lie,
	Peter Linss,
	Robert O'Callahan,
	Florian Rivoal,
	Alan Stearns,
	Steve Zilles,
	and all the rest of the
	<a href="http://lists.w3.org/Archives/Public/www-style/">www-style</a> community.