Title: CSS Overflow Module Level 4
Status: ED
Work Status: Revising
ED: https://drafts.csswg.org/css-overflow-4/
Shortname: css-overflow
Group: csswg
Level: 4
TR: https://www.w3.org/TR/css-overflow-4/
Editor: L. David Baron, Mozilla https://www.mozilla.org/, https://dbaron.org/, w3cid 15393
Editor: Florian Rivoal, On behalf of Bloomberg, http://florian.rivoal.net/, w3cid 43241
Editor: Elika J. Etemad / fantasai, Apple, http://fantasai.inkedblade.net/contact, w3cid 35400
Abstract: This module contains the features of CSS relating to scrollable overflow handling in visual media.
	It builds on the CSS Overflow Module Level 3,
	defining
	'line-clamp', its longhands, and its legacy pre-standard syntax;
	adding a 'block-ellipsis' property;
	and expanding 'overflow-clip-margin' with longhands.

type: dfn; spec:css-multicol-1; text:overflow column
spec:css-pseudo-4; type:selector; text:::first-letter
spec:css-pseudo-4; type:selector; text:::first-line
spec:css-writing-modes-4; type:dfn; text:start
spec:css-writing-modes-4; type:dfn; text:end

url: https://drafts.csswg.org/selectors-3/#subject; type: dfn; text: subject;
url: https://www.w3.org/TR/CSS21/visudet.html#strut; type: dfn; text: strut;

Introduction

This specification extends [[!CSS-OVERFLOW-3]]. It contains several primary sections: : [[#overflow-properties|Overflow Scrolling and Clipping Controls]] :: This section defines relatively simple extensions to the 'overflow-*' properties [[css-overflow-3#overflow-properties|in Level 3]]. : [[#auto-ellipsis|Automatic Ellipses]]. :: This section defines some experimental extensions to the '*-ellipsis' properties [[css-overflow-3#auto-ellipsis|in Level 3]]. Note: At the time of writing, [[CSS-OVERFLOW-3]] is not completely finalized yet. To avoid accidental divergences and maintenance overhead, This specification is written as a delta specification over css-overflow Level 3. Once the level 3 specification is final, its content will be integrated into this specification, which will then replace it. Until then, this specification only contains additions and extensions to level 3.

Value Definitions

This specification follows the CSS property definition conventions from [[!CSS2]] using the value definition syntax from [[!CSS-VALUES-3]]. Value types not defined in this specification are defined in CSS Values & 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 CSS-wide keywords as their property value. For readability they have not been repeated explicitly.

Module Interactions

This module extends features defined in [[!CSS-OVERFLOW-3]].

Overflow Concepts and Terminology

Issue: Copy [[css-overflow-3#overflow-concepts|Level 3 content]] when final.

Scrolling and Clipping Overflow

Issue: Copy [[css-overflow-3#overflow-properties|Level 3 content]] when final.

Managing Overflow: the 'overflow-x', 'overflow-y', and 'overflow' properties

This level extends the 'overflow-x' and 'overflow-y' properties (and 'overflow' [=shorthand=]) to apply to [=replaced elements=]. On [=replaced elements=], the [=used values=] of all computed values other than ''overflow/visible'' is ''overflow/clip''. Host languages should define [=UA style sheet=] rules that apply a default value of ''overflow/clip'' to such elements and set their 'overflow-clip-margin' to ''overflow-clip-margin/content-box''. Note: Application of 'overflow' to [=replaced elements=] was added to allow images to paint effects outside their effective layout box; the recommended UA style sheet rules are to effect their original behavior as a default. See discussion in Issue 7059 and Issue 7144. This is a change from CSS2.1 and is at-risk. ISSUE(7144): Application of 'overflow' to [=replaced elements=] is still being worked out.

Expanding Clipping Bounds: the 'overflow-clip-margin-*' properties

	Name: overflow-clip-margin-top, overflow-clip-margin-right, overflow-clip-margin-bottom, overflow-clip-margin-left, overflow-clip-margin-block-start, overflow-clip-margin-inline-start, overflow-clip-margin-block-end, overflow-clip-margin-inline-end
	Value: <> || <>
	Initial: ''0px''
	Inherited: no
	Logical property group: overflow-clip-margin
	Applies to: boxes to which 'overflow' applies
	Computed value: the computed <> and a <> keyword
	Animation type: per computed value if the <> values match; otherwise discrete
	
	Name: overflow-clip-margin, overflow-clip-margin-inline, overflow-clip-margin-block
	Value: <> || <>
	Initial: ''0px''
	Inherited: no
	Applies to: boxes to which 'overflow' applies
These properties and their shorthands define the overflow clip edge of the box, i.e. precisely how far 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=]. The [=shorthand=]/[=longhand=] relationships are as for 'margin', but note that the [=shorthands=] have a restricted syntax. Values are defined as follows:
: <> :: 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'' on non-[=replaced=] elements, or ''overflow-clip/content-box'' on [=replaced=] elements. ISSUE(7144): Application of 'overflow-clip-margin' to [=replaced elements=] is still being worked out. : <> :: 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.
The [=overflow clip edge=] is shaped in the corners exactly the same way as an outer box-shadow 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=].

Automatic Ellipses

Inline Overflow Ellipsis: the 'text-overflow' property

	Name: text-overflow
	Value: [ clip | ellipsis | <> | fade | <> ]{1,2}
	Initial: clip
	Applies to: block containers
	Inherited: no
	Percentages: refer to the width of the line box
	Computed value: as specified, with lengths made absolute
	Animation type: by computed value type
ISSUE: This section might need to be re-synced against [[CSS-OVERFLOW-3]]. This property specifies rendering when inline content overflows its line box edge in the inline progression direction of its block container element ("the block") that has 'overflow' other than ''overflow/visible''. Even though this property is not inherited, anonymous block container boxes generated to establish the line box's inline formatting context (see [=block container=]) are ignored, and the value of the property that applies is the one on the non anonymous box. This can be seen in the “nested paragraph” part of example 7: even though the word “NESTED” is wrapped in an anonymous block container whose 'text-overflow' property has the initial value, it is ellipsed. Text can overflow for example when it is prevented from wrapping (e.g. due to white-space: nowrap or a single word is too long to fit). Values have the following meanings:
clip
Clip inline content that overflows its block container element. Characters may be only partially rendered.
ellipsis
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.
<>
Render the given string to represent clipped inline content. The given string is treated as an independent paragraph for bidi purposes.
fade( [ <> ] )
Clip inline content that overflows its line box. Characters may be only partially rendered. In addition, the UA must apply a fade out effect near the edge of the line box, reaching complete transparency at the edge. Issue: Do we need to define the way the fade out is calculated so that the fading is identical across browsers? It should probably be something like ''mask-image: linear-gradient(to right, rgba(0,0,0,1), rgba(0,0,0,0))'', except applied to the relevant portion of the line only. The argument determines the distance over which the fade effect is applied. The <> is resolved against the width of the line box. Values lower than 0 are clipped to 0. Values greater than the width of the line box are clipped to the width of the line box. Issue: If the line box is too short to display the fade effect at the desired length, should we drop the effect, or shrink the distance it is applied over until it fits, or clip the end of the fade? Issue: How should we deal with things overflowing out of the line box, or overlapping onto it? Should fade apply to the logical content of the line, or to the physical area of the line box, or the intersection of both?
fade
Same as ''fade()'', but the distance over which the fading effect is applied is determined by the UA. ''1em'' is suggested as a reasonable value.
The term "character" is used in this property definition for better readability and means "grapheme cluster" [[!UAX29]] for implementation purposes. If there is one value, it applies only to the end line box edge. If there are two values, the first value applies to the line-left edge, and the second value applies to the line-right edge. The terms end, line-left and line-right are defined in [[!CSS-WRITING-MODES-3]]. Note: the use of line-left and line-right rather than start and end when there are two values is intentional, to facilitate the use of directional characters such as arrows. For the ellipsis and string values, implementations must hide characters and atomic inline-level elements at the applicable edge(s) of the line as necessary to fit the ellipsis/string, and place the ellipsis/string immediately adjacent to the applicable edge(s) of the remaining inline content. The first character or atomic inline-level element on a line must be clipped rather than ellipsed.

Bidi ellipsis examples

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

		div {
			font-family: monospace;
			white-space: pre;
			overflow: hidden;
			width: 9ch;
			text-overflow: ellipsis;
		}
Sample HTML fragments, renderings, and your browser:
HTMLReference renderingYour Browser
<div>שלום 123456</div>
123456 ם…
שלום 123456
<div dir=rtl>שלום 123456</div>
…456 שלום
שלום 123456

ellipsing details

user interaction with ellipsis

text-overflow examples

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

		div {
			font-family: Helvetica, sans-serif;
			line-height: 1.1;
			width: 3.1em;
			border: solid .1em black;
			padding: 0.2em; margin:1em 0;
		}
sample HTML fragments, renderings, and your browser:
HTML sample rendering your browser 

				<div>
				CSS IS AWESOME, YES
				</div>
First, a box with text drawing outside of it.
CSS IS AWESOME, YES

				<div style="text-overflow:clip; overflow:hidden">
				CSS IS AWESOME, YES
				</div>
Second, a similar box with the text clipped outside the box.
CSS IS AWESOME, YES

				<div style="text-overflow:ellipsis; overflow:hidden">
				CSS IS AWESOME, YES
				</div>
Third, a similar box with an ellipsis representing the clipped text.
CSS IS AWESOME, YES

				<div style="text-overflow:ellipsis; overflow:hidden">
				NESTED
				 <p>PARAGRAPH</p>
				WON'T ELLIPSE.
				</div>
Fourth, a box with a nested paragraph demonstrating anonymous block boxes equivalency and non-inheritance into a nested element.
NESTED

PARAGRAPH

WON'T ELLIPSE.

				<div style="text-overflow:fade; overflow:hidden">
				CSS IS AWESOME, YES
				</div>
a box with the text fading out on overflow.
CSS IS AWESOME, YES
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 (direction: rtl) block clips inline content on the left side, thus would place a text-overflow ellipsis on the left to represent that clipped content. Issue: insert RTL example diagram here to illustrate note.

ellipsis interaction with scrolling interfaces

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

		div.crawlbar {
			text-overflow: ellipsis;
			height: 2em;
			overflow: scroll;
			white-space: nowrap;
			width: 15em;
			border:1em solid black;
		}
sample HTML fragment: 

		<div class="crawlbar">
		CSS is awesome, especially when you can scroll
		to see extra text instead of just
		having it overlap other text by default.
		</div>
demonstration of sample CSS and HTML:
CSS is awesome, especially when you can scroll to see extra text instead of just having it overlap other text by default.
As some content is scrolled into view, it is likely that other content may scroll out of view on the other side. If that content's block container element is the same that's doing the scrolling, and the computed value of 'text-overflow' has two values, with the value applying to the start edge being a non-clip value, then implementations must render an ellipsis/string in place of the clipped content, with the same details as described in the value definition above, except that the ellipsis/string is drawn in the start (rather than end) of the block's direction (per the direction property). While the content is being scrolled, implementations may adjust their rendering of ellipses/strings (e.g. align to the box edges rather than line edges).
Same as previous example except with text-overflow: ellipsis ellipsis, demonstrated:
CSS is awesome, especially when you can scroll to see extra text instead of just having it overlap other text by default.
If there is insufficient space for both start and end ellipses/strings, then only the end ellipsis/string should be rendered.

Indicating Block-Axis Overflow: the 'block-ellipsis' property

		Name: block-ellipsis
		Value: none | auto | <>
		Initial: none
		Applies to: [=block containers=]
		Inherited: yes
		Percentages: N/A
		Computed value: specified value
		Animation type: discrete
This property allows inserting content into the last line box before a (forced or unforced) region break to indicate the continuity of truncated/interrupted content. It only affects line boxes contained directly by the block container 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 region break, then this property has no effect. Note: See [[css-overflow-4#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 block overflow ellipsis. Values have the following meanings:
none
The rendering is unaffected.
auto
Render an ellipsis character (U+2026)-- or a more typographically-appropriate equivalent-- as the block overflow ellipsis at the end of the affected line box. UAs should use the conventions of the content language, writing system, and writing mode to determine the most appropriate ellipsis string.
<>
Render the specified string as the block overflow ellipsis at the end of the affected line box. The UA may truncate this string if it is absurdly long.
When 'block-ellipsis' is not ''block-ellipsis/none'', the block overflow ellipsis string is wrapped in an anonymous inline and placed at the end of the line box as a direct child of the block container’s root inline box, 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 soft wrap opportunity (see [[!CSS-TEXT-3]]) that would still allow the entire block overflow ellipsis to fit on the line. For this purpose, soft wrap opportunities 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 alignment and justification occurs after placement, and measures the inserted block overflow ellipsis 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 fragmentation container in the [=fragmentation context=] that would receive subsequent content, then the content displaced by the block overflow ellipsis must be pushed to that fragmentation container. The UA must treat the block overflow ellipsis 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 block overflow ellipsis 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 min-content and max-content 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).

Fragmenting Overflow

Limiting Visible Lines: the 'line-clamp' shorthand property

		Name: line-clamp
		Value: none | <> || <<'block-ellipsis'>>
		Initial: none
		Percentages: N/A
The 'line-clamp' property is a shorthand 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:
none
Sets 'max-lines' to ''max-lines/none'', 'continue' to ''continue/auto'', and 'block-ellipsis' to ''block-ellipsis/none''.
<>
<'block-ellipsis'>
Sets 'continue' to ''discard'' if either or both values ares specified. Sets 'max-lines' to the specified <> or to ''max-lines/none'' if omitted, and 'block-ellipsis' to other component of the value if specified or to ''block-ellipsis/auto'' if omitted.
See the corresponding longhand properties for details about how this mechanism operates.
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)”: li { line-clamp: 5 "… (continued on next page)"; } strong { display: block; text-transform: uppercase; } <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> Sample rendering: 
			+---------------------------------------+
			| 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)  |
			+---------------------------------------+

Legacy compatibility

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. 
		Name: -webkit-line-clamp
		Value: none | <>
		Initial: none
		Percentages: N/A
	
		Name: continue
		New Values: -webkit-discard
Like 'line-clamp', '-webkit-line-clamp' is a shorthand of 'max-lines', 'continue', and 'block-ellipsis', except that: * its syntax is ''line-clamp/none'' | <> * it sets 'continue' to ''-webkit-discard'' instead of ''discard'' * it unconditionally sets 'block-ellipsis' to ''block-ellipsis/auto'' The -webkit-discard value behaves identically to ''discard'', except that it only takes effect if the [=specified value=] of the 'display' property is ''-webkit-box'' or ''-webkit-inline-box'' and the value of the '-webkit-box-orient' property 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 this blog post. 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.

Forcing a Break After a Set Number of Lines: the 'max-lines' property

		Name: max-lines
		Value: ''none'' | <>
		Initial: ''none''
		Applies to: block containers 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
This property only has an effect on boxes that are [=fragmentation containers=] that capture [=region breaks=]. Also, if the value of 'max-lines' is not none, a region break is forced after its Nth descendant in-flow line box, where N is the specified value of 'max-lines'. Only lines boxes in the same Block Formatting Context are counted: the contents of descendants that establish independent formatting contexts are skipped over while counting line boxes. If fewer than N line boxes exist, then 'max-lines' introduces no region break. 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 ignored. Note: The 'widows', 'orphans', and 'break-inside' properties do not affect the position of the forced region break introduced by the 'max-lines' property.
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 fragmentation containers). 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.

Fragmentation of Overflow: the 'continue' property

		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
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 region-fragment property from [[CSS-REGIONS-1]]. Once it is sufficiently stable in this specification, region-fragment should be removed from the regions specification in favor of this.
auto
If the box has more content than can fit, the excess content is handled according to the usual rules.
discard
The box must [=establish an independent formatting context=] and becomes a [=fragmentation container=] that captures region breaks, 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 forced (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.
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.
article with one excessively long line and four more that can fit in the block direction
''continue: discard'' ''continue: auto''
''overflow: visible'' rendering with continue:discard and overflow:visible rendering with continue:auto and overflow:visible
''overflow: hidden'' rendering with continue:discard and overflow:hidden rendering with continue:auto and overflow:hidden
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 Sydney F2F meeting. However, since intrinsic sizes are calculated across fragmentation containers, this content is taken into account for the purpose of finding the box’s min-content and max-content inline sizes (see [[css-break-3#varying-size-boxes]]). Min-content and max-content block sizes 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. Note: In the case of parallel fragmentation flows, content occurring after the fragmentation break in the box tree could still be rendered, if it is laid out above the position representing the end of this fragmentation container. Additionaliy, for compatibility (see [[#webkit-line-clamp]]), when the [=computed value=] of the 'continue' property is ''discard'' and the [=computed value=] of the '-webkit-box-orient' property is ''vertical'': * If the [=specified value=] of the 'display' property is ''-webkit-box'', the [=computed value=] becomes ''flow-root'' and the box establishes a [=BFC=]. * If the [=specified value=] of the 'display' property is ''-webkit-inline-box'', the [=computed value=] becomes ''inline-block'' and the box establishes a [=BFC=]. Note: This means that 'line-clamp' will work if both or neither of ''-webkit-box-orient: vertical'' and ''display: -webkit-box'' or ''display: -webkit-inline-box'' are set, but not if ''display: -webkit-box'' or ''display: -webkit-inline-box'' is set while ''-webkit-box-orient: vertical'' isn't, as the box would be a [=flex container=] rather than a [=block container=], and thus the 'continue' property would not apply.

Appendix A: Possible extensions for ''scrollbar-gutter''

This section is non-normative.
Issue: This section documents current attempts at extending the 'scrollbar-gutter' property to solve additional use cases. However, it does not currently have consensus. It is presented here to encourage discussion, but non-experimental implementation is not recommended.
This example exercises all the additional values of the 'scrollbar-gutter' property: * ''scrollbar-gutter: always'' on the scroll container * ''scrollbar-gutter: match-parent'' on each row inside the scroll container
With classic scrollbars
With overlay scrollbars
Name: scrollbar-gutter
New Values: auto | [ [ stable | always ] && both-edges? ] || match-parent
Applies to: all elements
For overlay scrollbars, the precise width of the scrollbar gutter, if present, is UA defined. However, it must not be 0, and it must not change based on user interactions with the page or the scrollbar even if the scrollbar itself changes, with the expectation that it covers the width of the overlay scrollbar in its widest form, to the extent that this is well defined. The new values of this property have the following meaning:
always
The scrollbar gutter is always present when 'overflow' is ''overflow/scroll'', ''overflow/hidden'', or ''overflow/auto'', regardless of the type of scrollbar or of whether the box is overflowing.
''scrollbar-gutter: always'' can be bused to solve the problem of (small) interactive elements near the edge of the element getting covered by an appearing overlay scrollbar. A representative case would be a basic todo list, with each line starting with some text and ending with a right-aligned checkbox. With a classic scrollbar, everything is fine, but an overlay scrollbar could obscure the check boxes and make them hard to interact with.
Checkboxes adjacent to a classic scrollbar
A scrollable todo list with checkboxes on the right edge, adjacent to the scrollbar. This situation poses no particular problem.
Checkboxes and an overlay scrollbar
A scrollable todo list with checkboxes on the right edge. When the overlay scrollbar is hidden, the situation poses no particular problem, but when it pops in, it covers the checkboxes, getting in the way of interacting with them.
Overlay Scrollbar are typically transient and disappear when not interacted with, so the checkboxes they cover are not impossible to use. But when the scrollbar is shown it does get in the way, and that makes for an awkward interaction. The author might try and solve the problem by adding some right padding, but (1) how much?, and (2) that padding isn't needed in the case of classic scrollbars. ''scrollbar-gutter: always'' solves this problem, yielding an identical result in first case of classic scrollbars, but adding the desired gutter with overlay scrollbars:
Checkboxes and an overlay scrollbar and ''scrollbar-gutter: always''
A scrollable todo list with checkboxes on the right edge, shifted left by a gutter. Whether the overlay scrollbar is hidden or visible, the checkboxes remain uncovered, and can be interacted with.
Apple is reluctant to add this value, as authors may use it too broadly, inserting gutters with overlay scrollbars even when not justified by interactive elements, defeating the space-saving advantage of overlay scrollbars. An alternative solution has been suggested: as the focus is interactive elements, maybe we could have a property that applies to the elements that needs to avoid being under the scrollbar. When turned on, it would enlarge the right or left margin of the element as appropriate by just the right value to push it out from under an overlay scrollbar if that's where it would end up, but would leave the element unchanged otherwise. Possibly, an addition toggle would cause the element to enlarge both its inline-end and inline-start margins or neither, rather than just one. This could typically be useful for block-level descendants of the scroller with visible borders or background: adding space on one side to avoid collisions with the overlay scrollbar would make them look off-center when the scrollbar disappears. Increasing the margin on both sides avoids that. Yet another possibility is to have a choice between growing the margin to protect the element, or growing the padding to protect the element's content. The syntax could be something like scrollbar-avoid: none | [self | content] && both-edges?. An interesting consideration is that this may alleviate the need for ''scrollbar-gutter: match-parent'', as it seems that situations that would have been addressed by ''scrollbar-gutter: stable'' or ''scrollbar-gutter: always'' on the parent and ''scrollbar-gutter: match-parent'' on the select children could instead be addressed by leaving the parent as ''scrollbar-gutter: auto'' and using scrollbar-avoid: self or scrollbar-avoid: content on the relevant children.
match-parent
On a [=block-level=] box whose parent has a [=scrollbar gutter=] (or gutters on both edges), this causes the box to have [=scrollbar gutter=] on the same side(s) and of the same width as its parent’s gutter(s). Moreover, that gutter is made to overlap that of the parent box.
The gutter of a ''scrollbar-gutter: match-parent'' box overlaps with that of its parent.
If the box with ''scrollbar-gutter: match-parent'' has a non-zero border or margin on the side where the gutter is expected, then the size of that box's gutter is parent.gutter - child.border - child.margin, and the gutter+border+margin is what collapses with the parent's gutter. If the box with ''scrollbar-gutter/ match-parent'' is itself a [=scroll container=], depending on the type of scrollbars, on its 'overflow' property, and on the other values of the 'scrollbar-gutter' property, it may need additional gutter for its own scrollbars. This comes in addition to the amount of gutter added for the sake of the ''scrollbar-gutter/match-parent'' value and does not collapse with the parent's gutter.
A scroller with an child whose background intrudes into the gutter, thanks to ''scrollbar-gutter/match-parent''
A ''scrollbar-gutter/match-parent'' box inside a scroll container with classic scrollbars, ''overflow: auto'', and ''scrollbar-gutter: stable''
The background of the match-parent element is visible in the gutter when the scrollbar isn't there.
A scrollable box with ''scrollbar-gutter/match-parent'' inside another scroll container
The element has a double gutter, one for its own scrollbar, one to match its parent's.
A scrollable box with ''scrollbar-gutter/match-parent'' inside another scroll container, with bidi
The element has a two gutters, one for its own scrollbar, one to match its parent's, on opposite sides.
A scrollable box with ''scrollbar-gutter:match-parent stable'' inside another scroll container, with bidi
The element has a two gutters, one for its own scrollbar (not shown, as it's not overflowing), one to match its parent's, on opposite sides.
Note: The following table summarizes the interaction of 'overflow' and 'scrollbar-gutter' for different types of scrollbars, showing in which case space is reserved for the scrollbar gutter.
Should space be reserved for the [=scrollbar gutter=]?
'overflow' 'scrollbar-gutter' Classic scrollbars Overlay scrollbars (whether overflowing or not)
Overflowing Not overflowing
''overflow/scroll'' ''scrollbar-gutter/auto'' yes yes
''scrollbar-gutter/stable'' yes yes
''scrollbar-gutter/always'' yes yes yes
''overflow/auto'' ''scrollbar-gutter/auto'' yes
''scrollbar-gutter/stable'' yes yes
''scrollbar-gutter/always'' yes yes yes
''overflow/hidden'' ''scrollbar-gutter/auto''
''scrollbar-gutter/stable'' yes yes
''scrollbar-gutter/always'' yes yes yes
''overflow/visible'', ''overflow/clip'' ''scrollbar-gutter/auto''
''scrollbar-gutter/stable'' if ''force'' if ''force''
''scrollbar-gutter/always'' if ''force'' if ''force'' if ''force''

Appendix B: Privacy Considerations

This specification introduces no new privacy considerations.

Appendix C: Security Considerations

This specification introduces no new security considerations.

Changes

Recent Changes

Significant changes since the June 2017 Working Draft include:

Changes Since Level 3

ISSUE: TBD

Acknowledgments

Thanks especially to the feedback from Rossen Atanassov, Bert Bos, Andreu Botella, Tantek Çelik, John Daggett, fantasai, Daniel Glazman, Vincent Hardy, Håkon Wium Lie, Peter Linss, Robert O'Callahan, Florian Rivoal, Alan Stearns, Steve Zilles, and all the rest of the www-style community.