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<pre class="metadata">
Title: CSS Overflow Module Level 5
Status: FPWD
Prepare for TR: yes
Work Status: Revising
Date: 2024-12-17
ED: https://drafts.csswg.org/css-overflow-5/
TR: https://www.w3.org/TR/css-overflow-5/
Shortname: css-overflow
Group: csswg
Level: 5
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
Editor: Robert Flack, Google, flackr@google.com, w3cid 98451
Abstract: This module contains the features of CSS relating to scrollable overflow handling in visual media.
It builds on the <a href="https://www.w3.org/TR/css-overflow-4/">CSS Overflow Module Level 4</a>,
adding the ability to generate and associate various scrolling controls
(markers to indicate scroll progress, buttons to trigger scrolling),
and adding an appendix containing an experimental exploration
of <a href="#fragmentation">redirecting overflow by fragmentation</a>.
</pre>
<pre class="link-defaults">
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
</pre>
<pre class=ignored-specs>
spec: css21
</pre>
<pre class="anchors">
url: https://www.w3.org/TR/CSS21/visudet.html#strut; type: dfn; text: strut;
</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;
}
.awesome-table td { padding: 5px; }
.awesome-table {
color: #000;
background: #fff;
margin: auto;
}
</style>
<h2 id="intro">
Introduction</h2>
This specification extends [[!CSS-OVERFLOW-4]].
It is currently a diff specification,
defining only a few new features;
see [[CSS-OVERFLOW-4]] for the rest of the features related to overflow.
: [[#scroll-navigation|Scroll navigation controls]]
::
This section defines the ability to associate [=scroll markers=]
with elements in a scroller
(or generate them automatically as ''::scroll-marker'' pseudo-elements,
with automatic user behavior and accessible labels),
which can be activated to scroll to the associated elements
and reflect the scroller's relative scroll progress
via the '':target-current'' pseudo-class.
It also defines ''::scroll-button()'' pseudo-elements,
which can be activated to cause their associated scroller
to scroll by a "page" in a given direction.
: [[#fragmentation|Redirection of Overflow]]
::
This section defines a highly experimental, exploratory new model
for handling overflow by redirecting it into newly-generated [=fragmentation containers=].
<h2 id="overflow-concepts">
Overflow Concepts and Terminology</h2>
Issue: Copy [[css-overflow-3#overflow-concepts|Level 3 content]] when final.
<h3 id="scrolling">Scrolling overflow</h3>
The following is added to the concepts in scrolling overflow:
Every [=scroll container=] maintains a <dfn export>current scroll target</dfn>.
It is initially <code>null</code>,
and is reset to <code>null</code> after any scrolling operations in that [=scroll container=]
initiated by the user,
or any script initiated operations that do not have a target,
or when the target is removed from the document.
Scrolling operations in the [=scroll container=] with a target Element or PseudoElement,
set the [=current scroll target=] to that target Element or PseudoElement.
Issue: Setting the current scroll target should be defined in how to <a>scroll a target into view</a>.
Issue: Use the current scroll target as an <a>anchor priority candidate</a> for scroll anchoring.
<h2 id="scroll-navigation">
Scroll navigation controls</h2>
<h3 id="scroll-markers">
Scroll markers</h3>
A <dfn>scroll marker</dfn> is any element or pseudo-element with a [=scroll target=].
An element or pseudo-element's <dfn>scroll target</dfn> is the {{Element}} indicated by the [=scroll marker=].
Which elements are [=scroll markers=], and what their [=scroll targets=] are, is host-language defined.
The [[html#the-a-element|HTML <a> element]] and [[svg2#Links|SVG <a> element]]
are [=scroll markers=],
whose [=scroll target=] is the <l spec=html>[=indicated part=]</l>.
While these navigational links can be created today,
there is little feedback to the user regarding the current content being viewed, and
the interaction model does not match the expectations of many modern accessible UI components.
This specification adds a mechanism for creating groups of [=scroll marker=]s,
and for automatically creating ''::scroll-marker'' pseudo-elements.
Within each group, the active marker reflects the current scroll position,
and can be styled to give the user an indication of which section they are in.
Use cases include a table of contents with links to relevant contents,
markers for scrolling carousel pages,
and scrollable tab panels.
Issue: Add images representing these examples.
<h4 id="scroll-marker-grouping">
Scroll marker grouping</h4>
An element with a <a href="https://open-ui.org/components/focusgroup.explainer/">focusgroup</a> attribute defines a <dfn>scroll marker group container</dfn>
having a <dfn>scroll marker group</dfn> containing all of the [=scroll marker=] elements for which this is the nearest ancestor [=scroll marker group container=].
Issue: The grouping of markers for scroll progress tracking should be separated from opting into focusgroup focus behavior.
A ''::scroll-marker-group'' pseudo-element is the [=scroll marker group container=] for its contained ''::scroll-marker'' pseudo-elements, which form a [=scroll marker group=] together.
<h4 id="scroll-marker-group-property">
The 'scroll-marker-group' property</h4>
<pre class=propdef>
Name: scroll-marker-group
Value: none | before | after
Initial: none
Applies to: [=scroll containers=]
Inherited: no
Computed value: specified value
Animation Type: discrete
Canonical Order: per grammar
</pre>
The 'scroll-marker-group' property specifies whether the [=scroll container=] should have a '::scroll-marker-group' pseudo-element created,
and its position relative to the scroll container.
<dl dfn-type=value dfn-for=scroll-marker-group>
<dt><dfn>none</dfn>
<dd>
The [=scroll container=] does not create a '::scroll-marker-group' pseudo-element.
<dt><dfn>before</dfn>
<dd>
The [=scroll container=] generates a ''::scroll-marker-group'' pseudo-element
whose box is an immediate preceding sibling to its [=originating element=].
<dt><dfn>after</dfn>
<dd>
The [=scroll container=] generates a ''::scroll-marker-group'' pseudo-element
whose box is an immediate following sibling to its [=originating element=].
</dl>
<h4 id="scroll-marker-group-pseudo">The ''::scroll-marker-group'' pseudo-element</h4>
The <dfn selector>::scroll-marker-group</dfn> [=fully styleable pseudo-element=]
is generated by a [=scroll container=] element
having a computed 'scroll-marker-group' property that is not ''scroll-marker-group/none''.
The ''::scroll-marker-group'' generates a box as a <em>sibling</em> of its [=originating element=],
either immediately preceding (if ''scroll-marker-group: before'')
or immediately following (if ''scroll-marker-group: after'').
The following additions are recommended for the default UA stylesheet
to ensure that the generation of scroll marker pseudo-elements does not invalidate the layout of the site:
<pre class="lang-css">
/* The generation of ::scroll-marker pseudo-elements shouldn't
* invalidate layout outside of this pseudo-element. */
::scroll-marker-group { contain: size !important; }
</pre>
The 'scroll-marker-group' implicitly behaves as a single focusable component,
establishing a <a href="https://open-ui.org/components/focusgroup.explainer/">focusgroup</a>.
<h4 id="scroll-marker-pseudo">The ''::scroll-marker'' pseudo-element</h4>
Similar to ''::before'' and ''::after'', all elements can have a ''::scroll-marker'' pseudo-element,
which is collected into the ''::scroll-marker-group'' of the nearest [=scroll container=] ancestor,
and scrolls to the element when activated.
When the computed 'content' value of a <dfn selector>::scroll-marker</dfn> pseudo-element is not ''content/none''
and its nearest ancestor [=scroll container=] [=scroll container=] has a computed 'scroll-marker-group' property that is not ''scroll-marker-group/none'',
the pseudo-element generates a box attached as a child of the ''::scroll-marker-group'' pseudo-element's generated box
on its nearest ancestor [=scroll container=].
These boxes are added in the [=tree order=] of their <a>originating element</a>.
These pseudo-elements have an indicated [=scroll target=] of their <a>originating element</a>.
They behave as an element with a <a href="https://html.spec.whatwg.org/multipage/interaction.html#attr-tabindex"><code>tabindex</code></a> of "-1",
making them focusable within their '::scroll-marker-group' either by arrow key navigation within the group,
or via the tab key when currently active or when no other ''::scroll-marker'' is active and this is the first marker in the group,
ensuring the group has a <a href="https://open-ui.org/components/focusgroup.explainer/#guaranteed-tab-stop">guaranteed tab stop</a>.
<h4 id="active-scroll-marker">
Selecting The Active Scroll Marker: the '':target-current'' pseudo-class</h4>
Exactly one [=scroll marker=] within each [=scroll marker group=] is determined to be active at a time.
Such "active" [=scroll markers=] match the <dfn selector>:target-current</dfn> pseudo-class.
<div class='example'>
The following snippet shows how the link to the currently scrolled section can be highlighted:
<pre highlight=css>
a:target-current {
font-weight: bold;
}
</pre>
</div>
A scrolling operation might animate towards a particular position
(e.g. scrollbar arrow clicks, arrow key presses, "behavior: smooth" programmatic scrolls)
or might directly track a user’s input
(e.g. touch scrolling, scrollbar dragging).
In either case, the user agent chooses an 'eventual scroll position' to which the scroller
will reach. This ensures that the relevant marker is activated immediately.
This 'eventual scroll position' is used to determine the active marker within each [=scroll marker group=].
Since markers themselves may represent just the start of the content (e.g. headers), we consider the active marker to be the first one which we are at or beyond the scroll position of.
Whenever a [=scroll container=] participating in one or more [=scroll marker groups=] is scrolled, or layout changes the eventual scroll position,
the user agent should determine and update the active marker for each [=scroll marker group=]
based on the section of content most likely to be of interest given the target scroll position.
<div algorithm="scroll tracking" class=example>
Example algorithm to determine the active marker for a given scroll marker <var>group</var>.
1. Let |scroller| be the nearest common ancestor [=scroll container=] of all of the [=scroll marker=] elements in |group|.
1. Let |active| be scroller.
1. While |active| is a [=scroll container=] containing [=scroll target=] elements targeted by |group|:
1. Let <var>scroller</var> be |active|.
1. Let <var>targets</var> be the set of the [=scroll target=] elements whose nearest ancestor [=scroll container=] is |scroller|
and the [=scroll container=] elements which contain [=scroll target=] elements targeted by the [=scroll marker group=] whose nearest ancestor [=scroll container=] is |scroller|.
1. : If |scroller| has a non-null [=current scroll target=]
::
Let active be the first item in |targets| encountered by a reverse tree order walk starting from the [=current scroll target=],
or the first item in tree order in |targets| if no target is found in the previous walk.
: Otherwise,
::
1. Let <var>primary</var> be the primary scrolling axis, assumed to be the block direction of the container's writing-mode.
1. Let <var>secondary</var> be the scrolling axis perpendicular to primary.
1. Let <var>position</var> be the 'eventual scroll position' considering ongoing scrolling operations.
1. For each <var>axis</var> of |primary|, followed by |secondary|:
1. Let <var>scrollport size</var> be the client size of |scroller| in the dimension |axis|.
1. For each |target| in |targets|, <a>determine the scroll-into-view position</a> of |target| in |axis|, storing this as the associated |target position| of |target|.
1. Let <var>scroll size</var> be the length of the <a>scrollable overflow area</a> of the |scroller| in the dimension |axis|.
1. Let <var>scroll range</var> be <code>|scroll size| - |scrollport size|</code>.
1. If |scroll range| is greater than 0, redistribute unreachable target positions:
1. Let <var>distribute range</var> be <code>min(1/8 * |scrollport size|, |scroll range| / 2)</code>.
1. Let <var>before targets</var> be all |targets| whose associated |target position| is less than <code>|distribute range|</code>.
1. Let <var>minimum position</var> be the minimum |target position| of |before targets|.
1. Update the associated |target position| of each target in |before targets| to
<code>(|target position| - |minimum position|) / (|distribute range| - |minimum position|) * |distribute range|</code>.
1. Let <var>after targets</var> be all |targets| whose associated |target position| is greater than <code>|scroll range| - |distribute range|</code>.
1. Let <var>maximum position</var> be the maximum |target position| of |after targets|.
1. Update the associated |target position| of each target in |after targets| to
<code>(|target position| - (|scroll range| - |distribute range|)) / (|maximum position| - (|scroll range| - |distribute range|)) * |distribute range| + (|scroll range| - |distribute range|)</code>.
1. Let |selected position| be the largest |target position|
where |target position| is equal to or before |position| in the |axis|,
or whose nearest smaller |target position| < |position| - |scrollport size| / 2 and whose |target position| < |position| + |scrollport size| / 2.
1. : If there is no such position,
::
Set the |selected position| to the first one.
1. Let |active| be the all of the |targets| whose associated |target position| is |selected position|.
1. Let |active| be the first item in |active| if it has more than one potential target.
1. Let |selected marker| be the [=scroll marker=] associated with |active|.
If multiple [=scroll marker=] elements are associated with |active|,
set |selected marker| to be the marker that is earliest in tree order among them.
1. Return |selected marker|
</div>
<div algorithm="update active marker">
Whenever the UA determines that a new marker is the <var>active marker</var> for a [=scroll marker group=] <var>group</var> it must run the following steps:
1. Set the active state of |active marker| to true.
1. : If |active marker| was the <a href="https://open-ui.org/components/focusgroup.explainer/#last-focused-memory">last-focused element</a> of the |group|,
::
Focus |active marker|
1. Set the <a href="https://open-ui.org/components/focusgroup.explainer/#last-focused-memory">last-focused element</a> of the |group| to |active marker|.
1. Set the active state of all other [=scroll marker=] elements in |group| to false.
</div>
<h4 id="scroll-marker-activation">Activation behavior</h4>
<div algorithm="scrollTargetElement activation">
When a [=scroll marker=] with a non-null [=scroll target=] is activated by explicit invocation or arrow key focus:
1. Let <var>element</var> be the [=scroll target=] of the control.
1. Let <var>block</var> be "<code>start</code>".
1. Let <var>inline</var> be "<code>start</code>".
1. <a lt='scroll a target into view'>Scroll the |element| into view</a> with <var>block</var> and <var>inline</var>.
1. : If the activation was triggered by invocation
::
1. <a spec=html>Follow the hyperlink</a> updating the URL, however retain focus on the marker element.
Note: If the user tabs away the focus behavior will ensure they tab into the relevant content.
</div>
<h4 id="scroll-marker-next-focus">Next tab-index-ordered focus</h4>
When a [=scroll marker=] is activated,
the next tabindex-ordered focus navigation will focus the [=scroll target=] if it is focusable,
otherwise, it will find the next focusable element from the [=scroll target=]
as though it were focused.
<h3 id="scroll-buttons">
Scroll Buttons</h3>
The <dfn>::scroll-button( '*' | <<scroll-button-direction>> )</dfn> pseudo-elements
are generated on [=scroll containers=]
when their computed 'content' value is not ''content/none''.
They generate boxes as if they were immediately preceding <em>siblings</em>
of their [=originating element=],
with content as specified by 'content'.
They exist after their [=originating element's=] ''::scroll-marker-group'' pseudo-element
(when used with ''scroll-marker-group: before'').
Four distinct ''::scroll-button()'' pseudo-elements can exist on a [=scroll container=],
each associated with a [=flow-relative=] direction,
based on their [=originating element's=] [=writing mode=]:
in order, [=block-start=], [=inline-start=], [=block-end=], and [=inline-end=].
The ''::scroll-button()'' pseudo-elements are both focusable and activatable by default,
with their activation behavior being to scroll their [=originating element=]
by one "page" in their associated direction,
similar to pressing PgUp/PgDn keys.
<span class=note>(Usually, this will be about 85% of the [=scrollport=] size.)</span>
Like those keys, this scroll has an [=intended direction and end position=].
Issue: [[CSSOM-VIEW-1]] will grow a "scroll by pages" algorithm,
at which point this can reference that.
See <a href="https://github.com/w3c/csswg-drafts/issues/10914#issuecomment-2380139977">
the resolution on issue #10914</a>.
Issue: Is this the best order for the buttons?
This does match the usual ordering of logical directions in CSS,
but similar buttons created manually
are usually either ordered with verticals and horizontals each grouped together
(aka block-start/block-end then inline-start/inline-end)
or the verticals "around" the horizontals
(aka block-start/inline-start/inline-end/block-end).
The four ''::scroll-button()'' pseudo-elements are individually selected
by the selector's argument.
A '*' arguments selects all four ''::scroll-button()''s;
otherwise the selected pseudo-element is determined
by the <dfn><<scroll-button-direction>></dfn> value:
<dl dfn-type=value dfn-for="<scroll-button-direction>, ::scroll-button()">
: <dfn>up</dfn>
: <dfn>down</dfn>
: <dfn>left</dfn>
: <dfn>right</dfn>
:: Selects the ''::scroll-button()'' corresponding to the given physical direction.
: <dfn>block-start</dfn>
: <dfn>block-end</dfn>
: <dfn>inline-start</dfn>
: <dfn>inline-end</dfn>
:: Selects the indicated ''::scroll-button()'' pseudo-element.
: <dfn>prev</dfn>
::
Selects either the [=block-start=] or [=inline-start=] ''::scroll-button()'',
whichever's axis has more "scrollable pages" in the [=originating element=]:
the [=originating element's=] [=scrollable overflow area|scrollable overflow height=]
divided by its [=scrollport=] height,
or the same but for widths.
If both dimensions are equally sized,
selects the [=block-start=] ''::scroll-button()''.
<p class=example>
For example, say the [=originating element=]
was 800px wide and 500px tall,
while its [=scrollable overflow area=]
was 1200px wide and 1000px tall.
The horizontal scrolling thus represents 1.5 "pages" (1200/800),
while the vertical scrolling represents 2 "pages" (1000/500),
so (assuming the element is in English)
the ''::scroll-button(prev)'' selector
would select the [=block-start=] button.
: <dfn>next</dfn>
:: Identical to ''::scroll-button()/prev'',
except it selects the [=block-end=] or [=inline-end=] ''::scroll-button()'' instead.
</dl>
Issue: Do we want to add some multi-button keywords
to make it easier to style several buttons the same way?
In particular, <css>all</css> is probably useful,
but maybe also <css>horizontal</css>/<css>vertical</css>/<css>block</css>/<css>inline</css>.
The ''::scroll-button()''s are [=fully styleable pseudo-elements=]: there is no restriction on what properties apply to them.
The '':disabled'' pseudo-class can apply to ''::scroll-button()''.
It matches a given button
when their [=originating element=]
can't be scrolled in their associated direction.
Issue: The UA stylesheet needs to specify that ''::scroll-button()''s
are styled identically to the <{button}> element,
including disabled styles.
Issue: Should ''::scroll-button()'' use the *full* UA styling of buttons,
aka ''appearance:button''?
Or the non-native rendering, aka ''appearance:none''?
If the former, we'll obviously need to define the interaction with 'appearance'.
<h3 id=overflow-pseudo-focus-behavior>
Focus behavior</h3>
The above features generate several focusable pseudo-elements.
This section defines some of the focus related behaviors.
<h4 id=active-element>
The active element</h4>
When a ''::scroll-button()'' or ''::scroll-marker'' is focused,
the {{DocumentOrShadowRoot/activeElement}}
is the [=scroll container=] the control is associated with.
<h4 id=focus-pseudoclasses>
Focus related pseudo-classes</h4>
When a ''::scroll-button()'' or ''::scroll-marker'' is focused,
none of the focus related pseudo-classes '':focus'', '':focus-visible'' and '':active'' match on the [=scrolling container=].
Instead, '':focus'' and, when relevant '':focus-visible'', match on the focused pseudo-element.
'':active'' matches on the pseudo-element while it is being activated.
'':focus-within'' matches on the [=scroll container=] and all of its ancestors in the [=flat tree=].
When a ''::scroll-marker'' is focused,
'':focus-within'' additionally matches on the ''::scroll-marker-group'' it belongs to.
<h4 id=focus-order>
Focus navigation order</h4>
While these pseudo-elements have a defined position in the element tree,
this isn't an optimal position for focus navigation
(aka "tab order")
for these controls.
Instead,
focus navigation between a [=scroll container=]
and the various pseudo-elements defined in this section
goes in the following order:
1. The ''::scroll-marker-group'' pseudo-elements of the [=scroll container=],
if it is set to ''scroll-marker-group: before''.
Note: The individual ''::scroll-marker'' pseudo-elements
generated by the [=scroll containers=] descendants
are reparented underneath this ''::scroll-marker-group'',
and navigated together as a "focus group".
2. The ''::scroll-button()'' pseudo-elements,
in the order they're defined as existing in.
3. The [=scroll container=] itself,
and its contents,
in the normal focus order they would be in.
4. The ''::scroll-marker-group'' pseudo-elements of the [=scroll container=],
if it is set to ''scroll-marker-group: after''.
<h2 id="fragmentation" class=no-num>
Appendix A: Redirection of Overflow</h2>
<!-- 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. -->
ISSUE: This section is highly experimental.
It documents current attempts
at extending the capabilities of the 'continue' 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.
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 was further refined in the CSS Overflow Module Level 3 [[CSS-OVERFLOW-3]].
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.
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.
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.
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 [[!CSS-BREAK-3]].
<h3 id="channelling-overflow">
Channeling Overflow: the 'continue' property</h3>
The 'continue' property gives authors the ability
to request that content that does not fit inside an element
be fragmented (in the sense of [[!CSS-BREAK-3]]),
and provides alternatives
for where the remaining content should continue.
Notably, this property explains traditional pagination,
and extends it further.
<pre class="propdef partial">
Name: continue
New Values: overflow | paginate | fragments
Initial: auto
Applies to: block containers [[!CSS2]], flex containers [[!CSS3-FLEXBOX]], and grid containers [[!CSS3-GRID-LAYOUT]]
Inherited: no
Percentages: N/A
Computed value: see below
Animation type: discrete
</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>auto
<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.
Issue: this is different from the definition in [[css-overflow-4#continue]],
where the specified value is the computed value.
Which is model better?
<dt><dfn>overflow</dfn>
<dd>Content that doesn't fit overflows, according to the 'overflow' property
<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:
1. On elements or pseudo elements with <a>layout containment</a> (see [[!CSS-CONTAIN-1]]),
if the specified value is ''continue/auto'' or ''continue/fragments''
then the computed value is ''continue/overflow''.
2. Otherwise, if the specified value is ''continue/auto''
1. 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''
2. On a page
the computed value is ''continue/paginate''
3. On a <a>fragment box</a>
the computed value is ''continue/fragments''
4. Otherwise, the computed value is ''continue/overflow''
3. Otherwise, if the specified value is ''continue/fragments''
1. On a page
the computed value is ''continue/paginate''
2. Otherwise, the computed value is the specified value
4. In all other cases, the computed value is the specified value
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>
<h3 id="paginated-overflow">
Paginated overflow</h3>
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 implementation 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="https://lists.w3.org/Archives/Public/www-style/2015Mar/0241.html">https://lists.w3.org/Archives/Public/www-style/2015Mar/0241.html</a>
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.
<h3 id="fragment-overflow">
Fragmented Overflow</h2>
This section introduces and defines the meaning of
the ''continue/fragments'' value of the 'continue' property.
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 [[!css-multicol-1]]
<span class="issue">though it defines <i>multi-column container</i></span>)
any content that would lead to the creation of <a>overflow columns</a> [[!css-multicol-1]]
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>
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.
<div class="example">
<table class="source-demo-pair">
<tr>
<td>
<pre><code highlight="html">
<!DOCTYPE HTML>
<title>Breaking content into
equal-sized cards</title>
<style>
.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;
}
</style>
<div class="in-cards">
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.
</div>
</code></pre>
<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>
</table>
</div>
<div class="example">
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,
it is more convenient to use the 'max-lines' property,
which 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.
<table class="source-demo-pair">
<tr>
<td>
<pre><code highlight="html">
<!DOCTYPE HTML>
<style>
.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;
}
</style>
<div class="article">
<i>...</i>
</div></code>
</pre>
<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>
</table>
</div>
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.
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.
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 [[CSS-REGIONS-1]],
and the work done on a model there,
and the editors of that specification,
should inform what happens in this specification.
<h3 id="fragment-styling">Fragment styling</h3>
<h4 id="fragment-pseudo-element">
The ''::nth-fragment()'' pseudo-element</h4>
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.
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.
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.
<h4 id="style-of-fragments">
Styling of fragments</h4>
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.)
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 [=selector/subject=]
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.
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>