drafts: css-overflow/Overview.bs@dc1f0ffba654 (annotated)

css-overflow/Overview.bs@dc1f0ffba654 (annotated)

css-overflow/Overview.bs

Sat, 14 Jan 2017 00:10:44 -0800

author: Florian Rivoal <[email protected]>
date: Sat, 14 Jan 2017 00:10:44 -0800
changeset 18674: dc1f0ffba654
parent 18647: d577d0a1ba38
child 18680: 078265332f8e
permissions: -rw-r--r--

Update personal info

 <h1>CSS Overflow Module Level 4</h1>
 <pre class="metadata">
 Status: ED
 Work Status: Exploring
 ED: https://drafts.csswg.org/css-overflow-4/
 Shortname: css-overflow
 Group: csswg
 Level: 4
 TR: https://www.w3.org/TR/css-overflow-4/
 Previous version: https://www.w3.org/TR/css-overflow-3/
 Editor: L. David Baron, Mozilla https://www.mozilla.org/, https://dbaron.org/
 Editor: Florian Rivoal, On behalf of Bloomberg, http://florian.rivoal.net/
 Abstract: This module contains the features of CSS relating to new mechanisms of overflow handling in visual media (e.g., screen or paper).  In interactive media, it describes features that allow the overflow from a fixed size container to be handled by pagination (displaying one page at a time).  It also describes features, applying to all visual media, that allow the contents of an element to be spread across multiple fragments, allowing the contents to flow across multiple regions or to have different styles for different fragments.
 !Change Log: <a href="https://hg.csswg.org/drafts/log/tip/css-overflow/Overview.bs">from 27 January 2015 to the present</a>
 !Change Log: <a href="https://hg.csswg.org/drafts/log/tip/css-overflow/Overview.src.html">from 28 March 2013 to 27 January 2015</a>
 !Change Log: <a href="https://hg.csswg.org/drafts/log/tip/css3-overflow/Overview.src.html">from 31 July 2012 to 27 March 2013</a>
 Ignored Terms: display-inside, display-outside
 </pre>
 <!-- FIXME: Regressions from bikeshed conversion: -->
 <!-- - Value lines in propdef tables no longer link to #values. -->
 <!-- - no longer says "Test suite: none yet" -->
 <!-- - Abstract has the most introductory sentence last -->
 <!-- FIXME: other bikeshed issues -->
 <pre class="link-defaults">
 spec:css-transforms-1; type:property; text:transform-style
 type: dfn; spec:css-multicol-1; text:overflow column
 </pre>
 <!-- FIXME: the break-* link doesn't actually work! -->
 <pre class="anchors">
 url: https://www.w3.org/TR/2008/CR-css3-marquee-20081205/#the-overflow-style; type: property; text: overflow-style;
 url: https://drafts.csswg.org/selectors-3/#subject; type: dfn; text: subject;
 </pre>
 	<style>
 		table.source-demo-pair {
 			width: 100%;
 		}
 		.in-cards-demo {
 			width: 13em;
 			height: 8em;
 			padding: 4px;
 			border: medium solid blue;
 			margin: 6px;
 			font: medium/1.3 Times New Roman, Times, serif;
 			white-space: nowrap;
 		}
 		.bouncy-columns-demo {
 			width: 6em;
 			height: 10em;
 			float: left;
 			margin: 1em;
 			font: medium/1.25 Times New Roman, Times, serif;
 			white-space: nowrap;
 		}
 		.bouncy-columns-demo.one {
 			background: aqua; color: black;
 			transform: rotate(-3deg);
 		}
 		.bouncy-columns-demo.two {
 			background: yellow; color: black;
 			transform: rotate(3deg);
 		}
 		.article-font-inherit-demo {
 			font: 1em/1.25 Times New Roman, Times, serif;
 			white-space: nowrap;
 		}
 		.article-font-inherit-demo.one {
 			width: 12em;
 			font-size: 1.5em;
 			margin-bottom: 1em;
 			height: 4em;
 		}
 		.article-font-inherit-demo.two {
 			width: 11em;
 			margin-left: 5em;
 			margin-right: 2em;
 		}
 		.dark-columns-demo {
 			width: 6em;
 			height: 10em;
 			float: left;
 			margin-right: 1em;
 			font: medium/1.25 Times New Roman, Times, serif;
 			white-space: nowrap;
 		}
 		.dark-columns-demo.one {
 			background: aqua; color: black;
 		}
 		.dark-columns-demo.one :link {
 			color: blue;
 		}
 		.dark-columns-demo.one :visited {
 			color: purple;
 		}
 		.dark-columns-demo.two {
 			background: navy; color: white;
 		}
 		.dark-columns-demo.two :link {
 			color: aqua;
 		}
 		.dark-columns-demo.two :visited {
 			color: fuchsia;
 		}
 		.article-max-lines-demo {
 			font: 1em/1.25 Times New Roman, Times, serif;
 			white-space: nowrap;
 		}
 		.article-max-lines-demo.one::first-letter {
 			font-size: 2em;
 			line-height: 0.9;
 		}
 		.article-max-lines-demo.one {
 			font-size: 1.5em;
 			width: 16em;
 		}
 		.article-max-lines-demo.two {
 			width: 11.5em;
 			float: left; margin-right: 1em;
 		}
 		.article-max-lines-demo.three {
 			width: 11.5em;
 			float: left;
 		}
 	</style>
 	<p>
 	</p>
 <h2 id="intro">
 Introduction</h2>
 	<p>
 		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.
 	</p>
 	<p>
 		CSS Level 2 [[CSS21]] 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.
 	</p>
 	<p>
 		However, scrolling is not the only way
 		to present large amounts of content,
 		and may even not be the optimal way.
 		After all, the codex replaced the scroll
 		as the common format for large written works
 		because of its advantages.
 	</p>
 	<p>
 		This specification introduces
 		a mechanism for Web pages to specify
 		that an element of a page should handle overflow
 		through pagination rather than through scrolling.
 	</p>
 	<p>
 		This specification also extends the concept of overflow
 		in another direction.
 		Instead of requiring that authors specify a single area
 		into which the content of an element must flow,
 		this specification allows authors to specify multiple fragments,
 		each with their own dimensions and styles,
 		so that the content of the element can flow from one to the next,
 		using as many as needed to place the content without overflowing.
 	</p>
 	<p>
 		In both of these cases, implementations must
 		break the content in the block-progression dimension.
 		Implementations must do this is described
 		in the CSS Fragmentation Module [[!CSS3-BREAK]].
 	</p>
 <h2 id="overflow-concepts">Types of overflow</h2>
 	<p>
 		CSS uses the term <dfn>overflow</dfn> to describe
 		the contents of a box
 		that extend outside that 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 overflow might be described as the 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 boxes and styles
 		of the box and of all its descendants whose containing block chain
 		<span class="issue">undefined term?</span>
 		includes the box.
 	</p>
 	<p>
 		In most cases, any of these types of overflow
 		can be computed for any box
 		from the bounds and properties of that box,
 		and from the overflow (of that type)
 		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, their descendants with
 		''transform-style: preserve-3d'' must also be examined.
 	</p>
 <h3 id="ink-overflow">Ink overflow</h3>
 	<p>
 		The <dfn id="ink-overflow0">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.
 	</p>
 	<p>
 		Since some effects in CSS (for example, the blurs in
 		'text-shadow' [[CSS3TEXT]] and 'box-shadow' [[CSS3BG]])
 		do not define what visual extent they cover, the extent
 		of the <a>ink overflow</a> is undefined.
 	</p>
 	<p class="issue">
 		Should we try to define it at all and just leave pieces undefined?
 	</p>
 	<p>
 		The <dfn>ink overflow region</dfn> is the non-rectangular region
 		occupied by the <a>ink overflow</a>, and the
 		<dfn>ink overflow rectangle</dfn> is
 		the minimal rectangle whose axis is aligned to the box's axes
 		and contains the <a>ink overflow region</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]].
 	</p>
 <h3 id="scrollable-overflow">Scrollable overflow</h3>
 	<p>
 		The <dfn id="scrollable-overflow0">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.
 	</p>
 	<p class="issue">
 		The following definition should be rewritten to use
 		the concept of <a href="https://drafts.csswg.org/css-transforms/#3d-rendering-context">3D rendering context</a> [[!CSS3-TRANSFORMS]]
 		and related terms,
 		particularly once those concepts stabilize following changes
 		proposed in the CSS WG meeting on the morning of 2014-01-28.
 	</p>
 	<p>
 		Given the following definitions
 		<span class="issue">which belong in [[CSS3-TRANSFORMS]]</span>:
 	</p>
 	<dl>
 		<dt><dfn>3d-preserving child</dfn></dt>
 		<dd>
 			A child box B of a containing block C is a 3d-preserving
 			child if it has ''transform-style: preserve-3d''
 			and the user-agent is not required to flatten it
 			based on the <a href="https://www.w3.org/TR/css3-transforms/#transform-style-property">requirements</a> in [[!CSS3-TRANSFORMS]].
 		</dt>
 		<dt><dfn>non-3d-preserving child</dfn></dt>
 		<dd>
 			A child C of a box P is a non-3d-preserving-child if
 			it is not a <a>3d-preserving child</a>.
 		</dd>
 		<dt><dfn>3d-preserving descendant</dfn></dt>
 		<dd>
 			Box D is a 3d-preserving descendant of box A if A is
 			an ancestor of D, and D and all of the boxes (if any)
 			in the containing block chain from D to A
 			are <a>3d-preserving child</a> boxes.
 		</dd>
 	</dl>
 	<p>The scrollable overflow of a box is the union of the following things,
 	all adjusted for transforms <span class="issue">undefined concept!</span> into the box's coordinate space:</p>
 	<ul>
 		<li>
 			for the box and all of its <a>3d-preserving descendant</a> boxes:
 			<ul>
 				<li>the box's own padding edge (for the box itself) or border edge (for <a>3d-preserving descendant</a> boxes)</li>
 				<li>the bounds <span class="issue">undefined term!</span> of any text directly in the box</li>
 				<li><span class="issue">MORE HERE!</span>
 			</ul>
 		<li>
 			for all the <a>non-3d-preserving child</a> boxes of the
 			box and its <a>3d-preserving descendant</a> boxes,
 			the scrollable overflow of the box
 		</li>
 	</ul>
 	<p class="issue">
 		I wrote this definition off the top of my head,
 		so it can't possibly be right.
 		It's missing tons of pieces!
 	</p>
 	<p class="issue">
 		The handling of preserve-3d subtrees here is probably wrong;
 		the elements should probably count
 		only towards the overflow of the element that flattens them.
 	</p>
 	<p>
 		The <dfn>scrollable overflow region</dfn> is the non-rectangular region
 		occupied by the <a>scrollable overflow</a>, and the
 		<dfn>scrollable overflow rectangle</dfn> is
 		the minimal rectangle whose axis is aligned to the box's axes
 		and contains the <a>scrollable overflow region</a>.
 		Note that the <a>scrollable 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]].
 	</p>
 <h2 id="overflow-properties">Overflow properties</h2>
 	<p>
 		The 'overflow-x' property specifies
 		the handling of overflow in the horizontal direction
 		(i.e., overflow from the left and right sides of the box),
 		and the 'overflow-y' property specifies the handling
 		of overflow in the vertical direction
 		(i.e., overflow from the top and bottom sides of the box)
 	</p>
 	<pre class=propdef>
 		Name: overflow-x, overflow-y
 		Value: ''visible'' | ''hidden'' | ''clip'' | ''scroll'' | ''auto''
 		Initial: ''visible''
 		Applies to: block containers [[!CSS21]], flex containers [[!CSS3-FLEXBOX]], and grid containers [[!CSS3-GRID-LAYOUT]]
 		Inherited: no
 		Percentages: N/A
 		Media: visual
 		Computed value: see below
 		Animatable: no
 		Canonical order: <abbr title="follows order of property value definition">per grammar</abbr>
 	</pre>
 	<p>
 		The 'overflow' property is a shorthand property
 		that sets the specified values of both 'overflow-x' and 'overflow-y'
 		to the value specified for 'overflow'.
 	</p>
 	<pre class=propdef>
 		Name: overflow
 		Value: ''visible'' | ''hidden'' | ''clip'' | ''scroll'' | ''auto''
 		Initial: see individual properties
 		Applies to: block containers [[!CSS21]], flex containers [[!CSS3-FLEXBOX]], and grid containers [[!CSS3-GRID-LAYOUT]]
 		Inherited: no
 		Percentages: N/A
 		Media: visual
 		Computed value: see individual properties
 		Animatable: no
 		Canonical order: <abbr title="follows order of property value definition">per grammar</abbr>
 	</pre>
 	<div id="overflow-computed-values">
 		<p>The computed values of 'overflow-x' and 'overflow-y'
 		are determined from the cascaded values [[!CSS3CASCADE]]
 		based on the following rules:</p>
 		<ol>
 			<li>
 				If one cascaded values is ''overflow/visible''
 				and the other is not,
 				then computed values are the cascaded values
 				with ''overflow/visible'' changed to ''overflow/auto''.
 			</li>
 			<li>
 				Otherwise, the computed values are as specified.
 			</li>
 		</ol>
 	</div>
 	<p>The values of these properties are:</p>
 	<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, it
 			may be rendered outside the block container.
 		</dd>
 		<dt><dfn>hidden</dfn>
 		<dd>
 			This value indicates that
 			the boxâ€™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 programatically,
 			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>Like ''hidden'',
 		this value indicates that
 		the content is clipped
 		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.
 		Issue: Mozilla implements -moz-hidden-unscrollable,
 		which is similar to ''clip'',
 		except that it does not cause the element to establish a BFC.
 		Should we match that?
 		<dt><dfn>scroll</dfn>
 		<dd>This value indicates that the content is clipped
 		and that 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 for a box
 		whether or not any of its content is clipped.
 		This avoids any problem with scrollbars appearing
 		and disappearing in a dynamic environment.
 		When this value is specified and the target medium is ''print'',
 		overflowing content may be printed.
 		<dt><dfn>auto</dfn>
 		<dd>The behavior of the ''overflow/auto'' value is user agent-dependent,
 		but should cause a scrolling mechanism to be provided for overflowing boxes.
 	</dl>
 	Even if 'overflow' is set to ''overflow/visible'',
 	content may be clipped to a UA's document window by the native operating environment.
 	If the computed value of 'overflow' is not ''overflow/visible'',
 	the element creates a block formatting context.
 	UAs must apply the 'overflow' property
 	set on the root element to the viewport.
 	HTML UAs must instead apply the â€˜overflowâ€™ property
 	from the <{body}> element to the viewport
 	if the value on the root element is ''visible''.
 	The ''visible'' value when used for the viewport
 	must be interpreted as ''overflow/auto''.
 	The element from which the value is propagated
 	must have a used value for 'overflow' of ''visible''.
 	In the case of a scrollbar being placed on an edge of the element's box,
 	it should be inserted between the inner border edge
 	and the outer padding edge.
 	Any space taken up by the scrollbars should be
 	taken out of (subtracted from the dimensions of)
 	the containing block formed by the element with the scrollbars.
 	Issue: import examples from [[CSS3-BOX]].
 	<p class="issue">
 		Explain which directions allow scrolling and which don't,
 		as a function of 'direction'
 		(including propagation of 'direction' to the ICB).
 	</p>
 	<p class="issue">
 		[[CSS3-MARQUEE]] describes an 'overflow-style' property,
 		but it has not picked up implementation experience
 		that the working group is aware of.
 		Should this document treat 'overflow-style' as a defunct proposal,
 		or should this document describe the 'overflow-style' property
 		and attempt to revive it,
 		despite that implementations have implemented
 		'overflow-x' and 'overflow-y' instead?
 	</p>
 <h2 id="fragmentation">Fragmentation of overflow</h2>
 The 'continue' property gives authors the ability
 to request that content that does not fit inside an element
 be fragmented (in the sense of [[!CSS3-BREAK]]),
 and provides alternatives
 for where the remaining content should continue.
 Notably, this property explains traditional pagination,
 and extends it further.
 	<pre class=propdef>
 		Name: continue
 		Value: ''auto'' | ''overflow'' | ''paginate'' | ''fragments'' | ''discard''
 		Initial: auto
 		Applies to: block containers [[!CSS21]], flex containers [[!CSS3-FLEXBOX]], and grid containers [[!CSS3-GRID-LAYOUT]]
 		Inherited: no
 		Percentages: N/A
 		Media: visual
 		Computed value: see below
 		Animatable: no
 		Canonical order: <abbr title="follows order of property value definition">per grammar</abbr>
 	</pre>
 Issue: The naming of this property and its values is preliminary.
 This was initially proposed as
 "fragmentation: auto | none | break | clone | page"
 in <a href="https://lists.w3.org/Archives/Public/www-style/2015Jan/0357.html">https://lists.w3.org/Archives/Public/www-style/2015Jan/0357.html</a>,
 and there is not yet wide agreement as to which naming is better.
 Issue: This property is meant to generalize and replace 'region-fragment'.
 Once it is sufficiently stable in this specification,
 'region-fragment' should be removed from the regions specification in favor of this.
 Note: ''continue: fragments'' replaces "overflow:fragments"
 from earlier versions of this specification,
 while ''continue: paginate'' replaces "overflow: paged-x | paged-y | paged-x-controls | paged-y-controls"
 	<dl dfn-for="continue" dfn-type="value">
 		<dt><dfn>auto</dfn>
 		<dd>''continue/auto'' may only occur as a computed value
 		if the element is a <a spec="css-regions">CSS Region</a>
 		other than the last one in a <a spec="css-regions">region chain</a>.
 		Content that doesn't fit is pushed to the next region of the chain.
 		In all other cases, ''continue/auto'' computes to one of the other values.
 		<dt><dfn>overflow</dfn>
 		<dd>Content that doesn't fit overflows, according to the 'overflow' property
 		<dt><dfn>discard</dfn>
 		<dd>Content that doesn't fit is discarded at a fragmentation break
 		Note: generalized from region-fragment: break; on the last region of a region chain
 		Issue: When the element isn't a <a spec="css-break">fragmentation container</a> already,
 		should this work by turning it directly into one,
 		or by creating a <a>fragment box</a> inside it like ''continue/fragments'' does?
 		<dt><dfn>paginate</dfn>
 		<dd>Content that doesn't fit paginates.
 		This creates a paginated view inside the element
 		similar to the way that 'overflow: scroll' creates a scrollable view.
 		See <a href="#paginated-overflow">paginated overflow</a>
 		Note: Print is effectively "continue: paginate" on the root.
 		<dt><dfn>fragments</dfn>
 		<dd>content that doesn't fit causes the element to copy itself and continue laying out.
 		See <a href="#fragment-overflow">fragment overflow</a>.
 	</dl>
 The computed value of the 'continue' for a given element or pseudo element is determined as follow:
 <ol>
 	<li>If the specified value is ''continue/auto''
 	<ol>
 		<li>On a <a spec="css-regions">CSS Region</a> other than the last one in a <a spec="css-regions">region chain</a>,
 		the computed value is ''continue/auto''
 		<li>On a page
 		the computed value is ''continue/paginate''
 		<li>On a <a>fragment box</a>
 		the computed value is ''continue/fragments''
 		<li>Otherwise, the computed value is ''continue/overflow''
 	</ol>
 	<li>If the specified value is ''continue/framgents''
 	<ol>
 		<li>On a page
 		the computed value is ''continue/paginate''
 		<li>Otherwise, the computed value is the specified value
 	</ol>
 	<li>In all other cases, the computed value is the specified value
 </ol>
 Issue: If we introduce a pseudo element that can select columns in a multicol,
 we would need to specify that auto computes to auto on it,
 or introduce a new value and have auto compute to that
 (but what would that value compute to on things that aren't columns?).
 Note: For background discussions leading to this property, see these threads:
 <a href="http://lists.w3.org/Archives/Public/www-style/2012May/1197.html">discussion of overflow, overflow-x, overflow-y and overflow-style</a> and
 <a href="https://lists.w3.org/Archives/Public/www-style/2015Jan/0357.html">proposal for a fragmentation property</a>
 <h2 id="paginated-overflow">Paginated overflow</h2>
 This section introduces and defines the meaning of the ''continue/paginate'' value of the 'continue' property.
 Issue: Write this section
 Issue: Pages should be possible to style with @page rules. How does that work for nested pages?
 <div class="issue">
 Should traditional pagination (e.g. when printing)
 be expressed through some magic in the computed value of ''continue/auto'',
 or by inserting this in the UA stylesheet:
 <pre><code class="lang-css">
 @media (overflow-block: paged), (overflow-block: optional-paged) {
 	:root {
 		continue: paginate;
 	}
 }
 </code></pre>
 </div>
 Issue: Traditional pagination (e.g. when printing) assumes that
 :root is contained in the page box,
 rather than having the page box be a pseudo element child of :root.
 Can we work around that using something similar to fragment boxes?
 Or maybe by having a fragment box (reproducing :root) inside a page box inside :root?
 Issue: How does the page box model work when it is a child of a regular css box?
 Issue: The initial proposal in [[CSS3GCPM]] and implemantation from Opera
 used 4 values instead of ''continue/paginate'':
 "paged-x | paged-y | paged-x-controls | paged-y-controls".
 Should this property also include these values,
 or are they better handled as separate properties?
 (e.g.: "pagination-layout: auto | horizontal | vertical", "pagination-controls: auto | none")
 Issue: Ability to display N pages at once
 rather than just one page at once?
 Could this be a value of "pagination-layout", such as:
 "pagination-layout: horizontal 2;"
 Issue: Brad Kemper has proposed a model for combining pagination and
 fragment overflow, which also deals with displaying multiple pages.
 <a href="http://www.w3.org/mid/[email protected]">http://www.w3.org/mid/[email protected]</a>
 	<p class="issue">
 		The current implementation of paginated overflow uses
 		the 'overflow'/'overflow-x'/'overflow-y' properties
 		rather than the 'overflow-style' property as proposed
 		in the [[CSS3GCPM]] draft
 		(which also matches the [[CSS3-MARQUEE]] proposal).
 		or the 'continue' property as described here.
 	</p>
 <h2 id="fragment-overflow">Fragment overflow</h2>
 This section introduces and defines the meaning of
 the ''continue/fragments'' value of the 'continue' property.
 	<p>
 		When the computed value of 'continue' for an element is ''continue/fragments'',
 		and implementations would otherwise have created a box for the element,
 		then implementations must create a sequence of <dfn>fragment box</dfn>es
 		for that element.
 		(It is possible for an element with ''continue: fragments''
 		to generate only one <a>fragment box</a>.
 		However, if an element's computed 'continue' is not ''continue/fragments'',
 		then its box is not a <a>fragment box</a>.)
 		Every <a>fragment box</a> is a fragmentation container,
 		and any overflow
 		that would cause that fragmentation container to fragment
 		causes another <a>fragment box</a> created as a next sibling
 		of the previous one.
 		<span class="issue">Or is it as though it's a next sibling of
 		the element?  Need to figure out exactly how this interacts with
 		other box-level fixup.</span>
 		Additionally, if the <a>fragment box</a> is also
 		a multi-column box (as defined in [[!CSS3COL]]
 		<span class="issue">though it defines <i>multi-column element</i></span>)
 		any content that would lead to the creation of <a>overflow columns</a> [[!CSS3COL]]
 		instead is flown into an additional fragment box.
 		However, fragment boxes may themselves be broken
 		(due to fragmentation in a fragmentation context outside of them,
 		such as pages, columns, or other fragment boxes);
 		such breaking leads to fragments of the same fragment box
 		rather than multiple fragment boxes.
 		(This matters because fragment boxes may be styled by their index;
 		such breaking leads to multiple fragments of a fragment box
 		with a single index.
 		This design choice is so that
 		breaking a fragment box across pages does not break
 		the association of indices to particular pieces of content.)
 		<span class="issue">Should a forced break that breaks to
 		an outer fragmentation context cause a new fragment of a single
 		fragment box or a new fragment box?</span>
 		<span class="issue">Should we find a term other than
 		<a>fragment box</a> here to make this a little less confusing?</span>
 	</p>
 	<p class="issue">
 		What if we want to be able to style the pieces of an element
 		split within another type of fragmentation context?
 		These rules prevent ever using ''::nth-fragment()'' for that,
 		despite that the name seems the most logical name for such a feature.
 	</p>
 	<div class="example">
 		<table class="source-demo-pair"><tr><td><pre>&lt;!DOCTYPE HTML&gt;
 &lt;title&gt;Breaking content into
   equal-sized cards&lt;/title&gt;
 &lt;style&gt;
   .in-cards {
     continue: fragments;
     width: 13em;
     height: 8em;
     padding: 4px;
     border: medium solid blue;
     margin: 6px;
     font: medium/1.3 Times New
       Roman, Times, serif;
   }
 &lt;/style&gt;
 &lt;div class="in-cards"&gt;
   In this example, the text in the div
   is broken into a series of cards.
   These cards all have the same style.
   The presence of enough content to
   overflow one of the cards causes
   another one to be created.  The second
   card is created just like it's the
   next sibling of the first.
 &lt;/div&gt;</pre></td><td>
 			<div class="in-cards-demo">In this example, the text in the<br>div is broken into a series of<br>cards.  These cards all have the<br>same style. The presence of<br>enough content to overflow<br>one of the cards causes another</div>
 			<div class="in-cards-demo">one to be created.  The second<br>card is created just like it's the<br>next sibling of the first.</div>
 		</td></tr></table>
 	</div>
 	<p class="issue">
 		We should specify that ''continue: fragments'' does not apply
 		to at least some table parts,
 		and perhaps other elements as well.
 		We need to determine exactly which ones.
 	</p>
 	<p class="issue">
 		This specification needs to say which type of
 		fragmentation context is created
 		so that it's clear which values of the 'break-*' properties
 		cause breaks within this context.
 		We probably want ''break-*: region'' to apply.
 	</p>
 	<p class="issue">
 		This specification needs a processing model
 		that will apply in cases where the layout containing the
 		fragments has characteristics that use the intrinsic size of the fragments
 		to change the amount of space available for them,
 		such as [[CSS3-GRID-LAYOUT]].
 		There has already been some work on such a processing model
 		in [[CSS3-REGIONS]],
 		and the work done on a model there,
 		and the editors of that specification,
 		should inform what happens in this specification.
 	</p>
 <h3 id="fragment-styling">Fragment styling</h3>
 <h4 id="fragment-pseudo-element">The ::nth-fragment() pseudo-element</h4>
 	<p>
 		The <dfn selector>::nth-fragment()</dfn> pseudo-element
 		is a pseudo-element
 		that describes some of the <a>fragment box</a>es generated by an element.
 		The argument to the pseudo-element takes the same syntax
 		as the argument to the :nth-child() pseudo-class
 		defined in [[!SELECT]], and has the same meaning
 		except that the number is relative to
 		<a>fragment box</a>es generated by the element
 		instead of siblings of the element.
 	</p>
 	<p class="note">
 		Selectors that allow addressing fragments
 		by counting from the end rather than the start
 		are intentionally not provided.
 		Such selectors would interfere with determining
 		the number of fragments.
 	</p>
 	<p class="issue">
 		Depending on future discussions,
 		this ''::nth-fragment(<var>an+b</var>)'' syntax
 		may be replaced with
 		the new ''::fragment:nth(<var>an+b</var>)'' syntax.
 	</p>
 <h4 id="style-of-fragments">Styling of fragments</h4>
 	<p class="issue">
 		Should this apply to continue:fragments only,
 		or also to continue:paginate?
 		(If it applies,
 		then stricter property restrictions would be needed
 		for continue:paginate.)
 	</p>
 	<p>
 		In the absence of rules with ''::nth-fragment()'' pseudo-elements,
 		the computed style for each <a>fragment box</a>
 		is the computed style for the element
 		for which the <a>fragment box</a> was created.
 		However, the style for a <a>fragment box</a> is also influenced
 		by rules whose selector's <a>subject</a> [[!SELECT]]
 		has an ''::nth-fragment()'' pseudo-element,
 		if the 1-based number of the <a>fragment box</a> matches
 		that ''::nth-fragment()'' pseudo-element
 		and the selector (excluding the ''::nth-fragment()'' pseudo-element)
 		matches the element generating the fragments.
 	</p>
 	<p>
 		When determining the style of the <a>fragment box</a>,
 		these rules that match the fragment pseudo-element
 		cascade together with the rules that match the element,
 		with the fragment pseudo-element adding the specificity
 		of a pseudo-class to the specificity calculation.
 		<span class="issue">Does this need to be specified in
 		the cascading module as well?</span>
 	</p>
 	<div class="example">
 		<table class="source-demo-pair"><tr><td><pre>&lt;!DOCTYPE HTML&gt;
 &lt;style&gt;
   .bouncy-columns {
     continue: fragments;
     width: 6em;
     height: 10em;
     float: left;
     margin: 1em;
     font: medium/1.25 Times New
       Roman, Times, serif;
   }
   .bouncy-columns::nth-fragment(1) {
     background: aqua; color: black;
     transform: rotate(-3deg);
   }
   .bouncy-columns::nth-fragment(2) {
     background: yellow; color: black;
     transform: rotate(3deg);
   }
 &lt;/style&gt;
 &lt;div class="bouncy-columns"&gt;
   <i>...</i>
 &lt;/div&gt;</pre></td><td>
 			<div class="bouncy-columns-demo one">In this<br>example, the<br>text in the div<br>is broken into<br>a series of<br>columns.  The<br>author<br>probably</div>
 			<div class="bouncy-columns-demo two">intended the<br>text to fill two<br>columns.  But<br>if it happens to<br>fill three<br>columns, the<br>third column is<br>still created.  It</div>
 			<div class="bouncy-columns-demo">just doesn't<br>have any<br>fragment-specific<br>styling because<br>the author<br>didn't give it<br>any.</div>
 		</td></tr></table>
 	</div>
 	<p>
 		Styling an ''::nth-fragment()'' pseudo-element with the 'continue'
 		property does take effect;
 		if a <a>fragment box</a> has a
 		computed value of 'continue' other than ''fragments''
 		then that fragment box is the last fragment.
 		However, overriding 'continue' on the first fragment
 		does not cause the <a>fragment box</a> not to exist;
 		whether there are fragment boxes at all is determined by
 		the computed value of overflow for the element.
 	</p>
 	<p>
 		Styling an ''::nth-fragment()'' pseudo-element with the 'content'
 		property has no effect;
 		the computed value of 'content' for the fragment box
 		remains the same as the computed value of content for the element.
 	</p>
 	<p>
 		Specifying ''display: none'' for a <a>fragment box</a> causes
 		the fragment box with that index not to be generated.
 		However, in terms of the indices
 		used for matching ''::nth-fragment()'' pseudo-elements
 		of later fragment boxes,
 		it still counts as though it was generated.
 		However, since it is not generated, it does not contain any content.
 	</p>
 	<p>
 		Specifying other values of 'display', 'position',
 		or 'float' is permitted, but is not allowed to change
 		the computed value of 'display-inside'.
 		(Since 'continue' only
 		applies to block containers, flex containers, and grid containers
 		the computed value of 'display-inside' is always
 		''display-inside/block'', ''display-inside/flex'', or
 		''display-inside/grid''.
 		<span class="issue">Need to specify exactly how this works,
 		but it depends on
 		having 'display-inside' and 'display-outside' specified.</span>
 	</p>
 	<p>
 		To match the model for other pseudo-elements
 		where the pseudo-elements live inside their corresponding element,
 		declarations in ''::nth-fragment()'' pseudo-elements override
 		declarations in rules without the pseudo-element.
 		The relative priority within such declarations is determined
 		by normal cascading order (see [[!CSS21]]).
 	</p>
 	<p>
 		Styles specified on ''::nth-fragment()'' pseudo-elements
 		do affect inheritance to content within the <a>fragment box</a>.
 		In other words, the content within the <a>fragment box</a> must
 		inherit from the fragment box's style (i.e., the pseudo-element style)
 		rather than directly from the element.
 		This means that elements split between fragment boxes may
 		have different styles for different parts of the element.
 	</p>
 	<p class="issue">
 		This inheritance rule allows specifying styles indirectly
 		(by using explicit ''inherit'' or using default inheritance
 		on properties that don't apply to ''::first-letter'')
 		that can't be specified directly
 		(based on the rules in the next section).
 		This is a problem.
 		The restrictions that apply to styling inside fragments
 		should also apply to inheritance from fragments.
 	</p>
 	<div class="example">
 		<table class="source-demo-pair"><tr><td><pre>&lt;!DOCTYPE HTML&gt;
 &lt;style&gt;
   .article {
     continue: fragments;
   }
   .article::nth-fragment(1) {
     font-size: 1.5em;
     margin-bottom: 1em;
     height: 4em;
   }
   .article::nth-fragment(2) {
     margin-left: 5em;
     margin-right: 2em;
   }
 &lt;/style&gt;
 &lt;div class="article"&gt;
   The &lt;code&gt;font-size&lt;/code&gt; property<i>...</i>
 &lt;/div&gt;</pre></td><td>
 			<div class="article-font-inherit-demo one">The <code>font-size</code> property<br>specified on the fragment<br>is inherited into the</div>
 			<div class="article-font-inherit-demo two">descendants of the fragment.<br>This means that inherited<br>properties can be used<br>reliably on a fragment, as in<br>this example.</div>
 		</td></tr></table>
 	</div>
 <h4 id="style-in-fragments">Styling inside fragments</h4>
 	<p class="issue">
 		Should this apply to continue:fragments only,
 		or also to continue:paginate?
 	</p>
 	<p>
 		The ''::nth-fragment()'' pseudo-element
 		can also be used to style
 		content inside of a <a>fragment box</a>.
 		Unlike the ''::first-line'' and ''::first-letter'' pseudo-elements,
 		the ''::nth-fragment()'' pseudo-element can be applied
 		to parts of the selector other than the subject:
 		in particular, it can match ancestors of the subject.
 		However, the only CSS properties applied
 		by rules with such selectors
 		are those that apply
 		to the ''::first-letter'' pseudo-element.
 	</p>
 	<p>
 		To be more precise,
 		when a rule's selector has ''::nth-fragment()'' pseudo-elements
 		attached to parts of the selector other than the subject,
 		the declarations in that rule apply to
 		a fragment (or pseudo-element thereof) when:
 	</p>
 	<ol>
 		<li>
 			the declarations are for properties that apply to the
 			''::first-letter'' pseudo-element,
 		</li>
 		<li>
 			the declarations would apply to
 			that fragment (or pseudo-element thereof)
 			had those ''::nth-fragment()'' pseudo-elements been removed,
 			with a particular association between
 			each sequence of simple selectors and the element it matched,
 			and
 		</li>
 		<li>
 			for each removed ''::nth-fragment()'' pseudo-element,
 			the fragment lives within a <a>fragment box</a>
 			of the element associated in that association
 			with the selector that the pseudo-element was attached to,
 			and whose index matches the pseudo-element.
 		</li>
 	</ol>
 	<div class="example">
 		<table class="source-demo-pair"><tr><td><pre>&lt;!DOCTYPE HTML&gt;
 &lt;style&gt;
   .dark-columns {
     continue: fragments;
     width: 6em;
     height: 10em;
     float: left;
     margin-right: 1em;
     font: medium/1.25 Times New
       Roman, Times, serif;
   }
   .dark-columns::nth-fragment(1) {
     background: aqua; color: black;
   }
   .dark-columns::nth-fragment(1) :link {
     color: blue;
   }
   .dark-columns::nth-fragment(1) :visited {
     color: purple;
   }
   .dark-columns::nth-fragment(2) {
     background: navy; color: white;
   }
   .dark-columns::nth-fragment(2) :link {
     color: aqua;
   }
   .dark-columns::nth-fragment(2) :visited {
     color: fuchsia;
   }
 &lt;/style&gt;
 &lt;div class="dark-columns"&gt;
   <i>...</i>
 &lt;/div&gt;</pre></td><td>
 			<div class="dark-columns-demo one">In this<br><a href="http://en.wiktionary.org/wiki/example">example</a>, the<br>text flows<br>from one<br>light-colored<br>fragment into<br>another<br>dark-colored</div>
 			<div class="dark-columns-demo two">fragment.  We<br>therefore want<br>different styles<br>for <a href="http://www.w3.org/Provider/Style/IntoContext.html">hyperlinks</a><br>in the different<br>fragments.</div>
 		</td></tr></table>
 	</div>
 <h3 id="max-lines">The 'max-lines' property</h3>
 	<p>
 		Authors may wish to style the opening lines of an element
 		with different styles
 		by putting those opening lines in a separate fragment.
 		However, since it may be difficult to predict the exact height
 		occupied by those lines
 		in order to restrict the first fragment to that height,
 		this specification introduces a 'max-lines' property
 		that forces a fragment to break
 		after a specified number of lines.
 		This forces a break after the given number of lines
 		contained within the element or its descendants,
 		as long as those lines are in the same block formatting context.
 	</p>
 	<pre class=propdef>
 		Name: max-lines
 		Value: ''none'' | ''&lt;integer>''
 		Initial: ''none''
 		Applies to: fragment boxes
 		Inherited: no
 		Animatable: as <a href="https://www.w3.org/TR/css3-transitions/#animatable-types">integer</a>
 		Percentages: N/A
 		Media: visual
 		Computed value: specified value
 		Canonical order: <abbr title="follows order of property value definition">per grammar</abbr>
 	</pre>
 	<dl dfn-for="max-lines" dfn-type="value">
 		<dt><dfn>none</dfn>
 		<dd>
 			<p>
 				Breaks occur only as specified elsewhere.
 			</p>
 		</dd>
 		<dt><dfn><<integer>></dfn>
 		<dd>
 			<p>
 				In addition to any breaks specified elsewhere,
 				a break is forced before any line that would exceed
 				the given number of lines
 				being placed inside the element
 				(excluding lines that are in
 				a different block formatting context from
 				the block formatting context to which
 				an unstyled child of the element would belong).
 			</p>
 			<p class="issue">
 				If there are multiple boundaries between this line
 				and the previous, where exactly (in terms of element
 				boundaries) is the break forced?
 			</p>
 			<p>
 				Only positive integers are accepted.
 				Zero or negative integers are a parse error.
 			</p>
 		</dd>
 	</dl>
 <p class="issue">Should this apply to fragment overflow only, or also
 to pagination?
 Given what we're doing with the continue property,
 it should actually apply to any fragmentainer.</p>
 Issue: having max-lines do nothing on regular elements is not ideal.
 When applied to non fragmentainers,
 it should probably cause 'continue' to compute to ''continue/discard''
 so that you only need to reach for one property rather than 2 to get
 that effect.
 	<div class="example">
 		<table class="source-demo-pair"><tr><td><pre>&lt;!DOCTYPE HTML&gt;
 &lt;style&gt;
   .article {
     continue: fragments;
   }
   .article::first-letter {
     font-size: 2em;
     line-height: 0.9;
   }
   .article::nth-fragment(1) {
     font-size: 1.5em;
     max-lines: 3;
   }
   .article::nth-fragment(2) {
     column-count: 2;
   }
 &lt;/style&gt;
 &lt;div class="article"&gt;
   <i>...</i>
 &lt;/div&gt;</pre></td><td>
 			<div class="article-max-lines-demo one">The max-lines property allows<br>authors to use a larger font for the first<br>few lines of an article.  Without the</div>
 			<div class="article-max-lines-demo two">max-lines property, authors<br>might have to use the<br>'height' property instead, but<br>that would leave a slight gap<br>if the author miscalculated<br>how much height a given<br>number of lines would<br>occupy (which might be</div>
 			<div class="article-max-lines-demo three">particularly hard if the author<br>didn't know what text would<br>be filling the space, exactly<br>what font would be used, or<br>exactly which platform's font<br>rendering would be used to<br>display the font).</div>
 		</td></tr></table>
 	</div>
 <h2 id="static-media">Overflow in static media</h2>
 	<p class="issue">
 		This specification should define useful behavior
 		for all values of 'overflow' and 'continue'
 		in static media (such as print).
 		Current implementation behavior is quite poor and
 		produces unexpected results when authors have not considered
 		what will happen when
 		the content they produce for interactive media
 		is printed.
 	</p>
 <h2 class=no-num id="acknowledgments">
 Acknowledgments</h2>
 	<p>
 		Thanks especially to the feedback from
 		Rossen Atanassov,
 		Bert Bos,
 		Tantek Ã‡elik,
 		John Daggett,
 		fantasai,
 		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.
 	</p>

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css-overflow/Overview.bs