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<pre class="metadata">
Title: CSS Overflow Module Level 3
Status: ED
Work Status: Revising
ED: https://drafts.csswg.org/css-overflow-3/
Shortname: css-overflow
Group: csswg
Level: 3
TR: https://www.w3.org/TR/css-overflow-3/
Previous version: https://www.w3.org/TR/2021/WD-css-overflow-3-20211223/
Former Editor: L. David Baron, Mozilla https://www.mozilla.org/, https://dbaron.org/, w3cid 15393
Editor: Elika J. Etemad / fantasai, Apple, http://fantasai.inkedblade.net/contact, w3cid 35400
Editor: Florian Rivoal, On behalf of Bloomberg, http://florian.rivoal.net/, w3cid 43241
Abstract: This module contains the features of CSS relating to scrollable overflow handling in visual media.
This level is focused on completing a precise specification for the existing overflow features,
including the 'overflow' property and its longhands;
and the 'text-overflow' property.
A few additional features introduced in support of [[CSS-CONTAIN-1]] and [[CSS-CONTAIN-2]] are also defined:
''overflow: clip'' and the 'overflow-clip-margin' property.
Ignored Vars: B, P
Ignored Terms:viewport, flex order, -webkit-box-orient
At Risk: the 'max-lines' property
Status Text:
The description of 'overflow' and its longhands is considered
significantly more complete and correct than previous working drafts
or than [[CSS2]],
but a few questions and issues remain open.
''overflow: clip' and 'overflow-clip-margin' are rather new,
and lack implementation experience.
'text-overflow' is stable,
is unchanged form its earlier definition in [[CSS-UI-3]].
While not yet fully validated by implementation experience,
the design of 'line-clamp' and its longhands is considered roughly complete.
Earlier versions of this specification included experimental new ideas
for handling overflow by creating new boxes using fragmentation.
These ideas are not abandoned; they are merely <a href="https://drafts.csswg.org/css-overflow-4/">deferred until Level 4</a>.
Work will resume on fragmented overflow once this level stabilizes completed.
</pre>
<!-- FIXME: Regressions from bikeshed conversion: -->
<!-- - Value lines in propdef tables no longer link to #values. -->
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<pre class="link-defaults">
spec:css-transforms-2; type:property; text:transform-style
type: dfn; spec:css-multicol-1; text:overflow column
type: dfn; spec:css-transforms-2; text:3d rendering context
spec:css-writing-modes-4; type:dfn; text:end
spec:css-pseudo-4; type:selector; text:::first-line
spec:css-pseudo-4; type:selector; text:::first-letter
spec:css-display-3; type:property; text:display
spec:css-sizing-3; type:property; text:max-height
spec:css-box-4; type:dfn; text:content area
spec:mediaqueries-3; type:value; text:print
spec:html; type:method; text:focus()
</pre>
<!-- 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://www.w3.org/TR/CSS22/visuren.html#positioned-element; type: dfn; spec:css2; text:positioned;
url: https://www.w3.org/TR/CSS22/visudet.html#strut-element; type: dfn; spec:css2; text:strut;
</pre>
<style>
.awesome-table td { padding: 5px; }
.awesome-table {
color: #000;
background: #fff;
margin: auto;
}
</style>
<h2 id="intro">
Introduction</h2>
In CSS Level 1 [[CSS1]], placing more content than would fit
inside an element with a specified size
was generally an authoring error.
Doing so caused the content to extend
outside the bounds of the element,
which would likely cause
that content to overlap with other elements.
CSS Level 2 [[CSS2]] introduced the 'overflow' property,
which allows authors to have overflow be handled by scrolling,
which means it is no longer an authoring error.
It also allows authors to specify
that overflow is handled by clipping,
which makes sense when the author's intent
is that the content not be shown.
This specification introduces the long-standing de-facto 'overflow-x' and 'overflow-y' properties,
adds a ''overflow/clip'' value,
and defines overflow handling more fully.
[Something something 'max-lines'.]
Note: This specification also reproduces the definition of the 'text-overflow' property
previously defined in [[CSS-UI-3]],
with no addition or modification,
in order to present 'text-overflow' and 'block-ellipsis' together.
<h3 id="values">
Value Definitions</h3>
This specification follows the <a href="https://www.w3.org/TR/CSS2/about.html#property-defs">CSS property definition conventions</a> from [[!CSS2]]
using the <a href="https://www.w3.org/TR/css-values-3/#value-defs">value definition syntax</a> from [[!CSS-VALUES-3]].
Value types not defined in this specification are defined in CSS Values & Units [[!CSS-VALUES-3]].
Combination with other CSS modules may expand the definitions of these value types.
In addition to the property-specific values listed in their definitions,
all properties defined in this specification
also accept the <a>CSS-wide keywords</a> as their property value.
For readability they have not been repeated explicitly.
<h3 id="placement">
Module Interactions</h3>
This module replaces (supersedes) and extends features defined in [[!CSS2]] section
<a href="https://www.w3.org/TR/CSS2/visufx.html#overflow-clipping">11.1 Overflow and clipping</a>
and [[!CSS-UI-3]] section
<a href="https://www.w3.org/TR/css-ui-3/#text-overflow">5.2. Overflow Ellipsis: the text-overflow property</a>.
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<h2 id="overflow-concepts">
Overflow Concepts and Terminology</h2>
CSS uses the term <dfn export>overflow</dfn> to describe
the contents of a box
that extend outside one of that box's edges
(i.e., its <i>content edge</i>, <i>padding edge</i>,
<i>border edge</i>, or <i>margin edge</i>).
The term might be interpreted as
elements or features that cause this overflow,
the non-rectangular region occupied by these features,
or, more commonly, as
the minimal rect
F438
angle that bounds that region.
A box's overflow is computed based on the layout and styling of the box itself
and of all descendants whose <a>containing block chain</a>
includes the box.
In most cases, <a>overflow</a>
can be computed for any box
from the bounds and properties of that box itself,
plus the <a>overflow</a>
of each of its children.
However, this is not always the case; for example,
when ''transform-style: preserve-3d'' [[CSS3-TRANSFORMS]] is used on
some of the children, any of their descendants with
''transform-style: preserve-3d'' must also be examined.
There are two different types of overflow,
which are used for different purposes by the UA:
<ul>
<li><a>ink overflow</a>
<li><a>scrollable overflow</a>
</ul>
<h3 id="ink">Ink Overflow</h3>
The <dfn export>ink overflow</dfn> of a box
is the part of that box and its contents that
creates a visual effect outside of
the box's border box.
Ink overflow is the overflow of painting effects defined to not affect layout
or otherwise extend the <a>scrollable overflow area</a>,
such as <a href="https://www.w3.org/TR/css-backgrounds/#box-shadow">box shadows</a>,
<a href="">border images</a>,
<a href="https://www.w3.org/TR/css-text-decor-3/">text decoration</a>,
overhanging glyphs (with negative side bearings,
or with ascenders/descenders extending outside the em box),
<a href="https://www.w3.org/TR/css-ui-3/#outline-props">outlines</a>,
etc.
Since some effects in CSS (for example, the blurs in
'text-shadow' [[CSS-TEXT-3]] and 'box-shadow' [[CSS-BACKGROUNDS-3]],
which are theoretically infinite)
do not define what visual extent they cover, the extent
of the <a>ink overflow</a> is undefined.
The <dfn export lt="ink overflow region | ink overflow area">ink overflow area</dfn> is the non-rectangular area
occupied by the <a>ink overflow</a>
of a box and its contents,
and the <dfn export>ink overflow rectangle</dfn> is
the minimal rectangle whose axes are aligned to the box's axes
and that contains the <a>ink overflow area</a>.
Note that the <a>ink overflow rectangle</a> is a rectangle
in the box's coordinate system, but might be non-rectangular
in other coordinate systems due to transforms. [[CSS3-TRANSFORMS]]
Any overflow of [=replaced=] content is always [=ink overflow=]
(as opposed to [=scrollable overflow=]).
<h3 id="scrollable">
Scrollable Overflow</h3>
The <dfn export>scrollable overflow</dfn> of a box is the
set of things extending outside of that box's padding edge
for which a scrolling mechanism needs to be provided.
The <dfn export lt="scrollable overflow region | scrollable overflow area">scrollable overflow area</dfn> is the non-rectangular region
occupied by the <a>scrollable overflow</a>, and the
<dfn export>scrollable overflow rectangle</dfn> is
the minimal rectangle whose axes are aligned to the box's axes
and that contains the <a>scrollable overflow area</a>.
The <a>scrollable overflow area</a> is the union of:
<ul>
<li>
The [=scroll container=]’s own [=padding box=].
<li>
All [=line boxes=] directly contained by the [=scroll container=].
<li>
The border boxes
of all boxes for which it is the containing block
and whose border boxes are positioned not wholly
in the [=negative scrollable overflow region=],
accounting for transforms by projecting each box onto
the plane of the element that establishes its <a>3D rendering context</a>.
[[!CSS3-TRANSFORMS]]
Issue: Is this description of handling transforms
sufficiently accurate?
Border boxes with zero area
do not affect the [=scrollable overflow area=].
<li>
The margin areas of <a>grid item</a> and <a>flex item</a> boxes
for which the box establishes a containing block.
The UA may <em>additionally</em> include
the margin areas of other boxes
for which the box establishes a containing block;
however,
the conditions under which such margin areas are included
is undefined in this level.
<span class="issue">This needs further testing and investigation;
is therefore deferred in this draft.</span>
<li>
The <a>scrollable overflow areas</a> of all of the above boxes
(including zero-area boxes
and accounting for transforms as described above),
provided they themselves have ''overflow: visible''
(i.e. do not themselves trap the overflow)
and that <a>scrollable overflow</a> is not already clipped
(e.g. by the 'clip' property or the 'contain' property).
Note: The 'mask-*' properties [[!CSS-MASKING-1]]
do not affect the <a>scrollable overflow area</a>.
ISSUE(8607): Need to evaluate what should/should not clip [=scrollable overflow=].
<li>
Additional padding added
to the [=end=]-side of the [=scrollable overflow rectangle=]
as necessary to enable a scroll position
that satisfies the requirements of ''place-content: end'' alignment.
Note: This padding represents,
within the [=scrollable overflow rectangle=],
the box’s own padding
so that when its content is scrolled to the end,
there is padding between the end-edge of its in-flow (or floated) content
and the border edge of the box.
It typically ends up being exactly the same size as the box's own padding,
except in a few cases--
such as when an out-of-flow positioned element,
or the visible overflow of a descendent,
has already increased the size of the [=scrollable overflow rectangle=]
outside the conceptual “content edge” of the [=scroll container=]’s content.
<figure>
<img src="images/scroll-align-padding.jpg">
<figcaption>
Issue: Replace this image with a proper SVG.
</figcaption>
</figure>
</ul>
Additionally, due to Web-compatibility constraints
(caused by authors exploiting legacy bugs to surreptitiously hide content from visual readers but not search engines and/or speech output),
UAs must clip any content in the [=negative scrollable overflow region=]
(thereby behaving as if they had no <a>scrollable overflow</a>
on the wrong side of the [=scroll origin=]).
<!--
Considerations:
* floats
* relpos
* margins that collapsed through to the edge of the scrollable area
* inline-side margins on both blocks and inlines
Notes:
The reasoning for only including border boxes is to minimize the cases
in which overflow scrolling is triggered; however once overflow is triggered,
excluding margins creates tight, uncomfortable edges
because the margin has been effectively truncated.
Testcase:
http://software.hixie.ch/utilities/js/live-dom-viewer/?%3C!DOCTYPE%20html%3E%0A%3Cstyle%3E%0A%20%20div%20%7B%20height%3A%2050px%3B%20overflow%3A%20scroll%3B%20border%3A%20solid%3B%20position%3A%20relative%3B%7D%0A%20%20p%20%7B%20margin%3A%2025px%3B%20background%3A%20red%3B%20%7D%0A%20%20q%20%7B%20position%3A%20absolute%3B%20background%3A%20blue%3B%20top%3A%2025px%3B%20margin%3A%2025px%3B%20%7D%0A%3C%2Fstyle%3E%0A%3Cdiv%3E%0A%20%20%3Cp%3ETest%20%3Cq%3ETest2%0A%3C%2Fdiv%3E
-->
Note: The <a>scrollable overflow rectangle</a> is always a rectangle
in the box's own coordinate system, but might be non-rectangular
in other coordinate systems due to transforms [[CSS3-TRANSFORMS]].
This means scrollbars can sometimes appear when not actually necessary.
<h3 id="scrolling">
Scrolling Overflow</h3>
A box’s [=overflow=] can be visible or clipped.
CSS also allows a box to be a <dfn export>scroll container</dfn>
that allows clipped parts of its [=scrollable overflow area=]
to be scrolled into view.
The visual “viewport” of a <a>scroll container</a>
(through which the <a>scrollable overflow area</a> can be viewed)
coincides with its padding box,
and is called the <dfn export>scrollport</dfn>.
A box’s <dfn export>nearest scrollport</dfn>
is the [=scrollport=] of its nearest [=scroll container=] ancestor.
Scrolling operations can be initiated by the user
(for example, by manipulating a scrollbar, swiping a touchscreen, or using keyboard controls)
or by script
(<span class=non-normative>for example, by the {{scrollIntoView()}} or {{focus()}} APIs</span>).
The initial position of the [=scrollable overflow rectangle=] within the [=scrollport=]
before any scrolling operations take effect is the <dfn export>initial scroll position</dfn>.
The [=initial scroll position=]
is typically dependent on the [=scroll container=]’s [=writing mode=],
and, unless otherwise specified,
coincides with its [=scroll origin position=].
However, the 'align-content' and 'justify-content' properties [[!CSS-ALIGN-3]]
can be used to change the [=initial scroll position=],
see [[css-align-3#overflow-scroll-position]].
A <dfn export>scroll position</dfn> is a particular alignment
of the [=scrollable overflow rectangle=]
within its [=scrollport=].
It is associated with a <dfn export>scroll offset</dfn>
which is its distance from the [=scroll origin=].
The <dfn export>scroll origin</dfn> is the anchor coordinate of the [=scrollable overflow rectangle=],
from which the [=scrollable overflow rectangle=] expands.
Unless otherwise specified,
it is the [=block-start=] [=inline-start=] corner of the [=scrollable overflow rectangle=].
(For example, in a [=flex container=] it is the [=main-start=] [=cross-start=] corner.)
The area beyond the [=scroll origin=] in either axis
is considered the <dfn export>negative scrollable overflow region</dfn>:
content rendered here is not accessible to the reader,
see [[#scrollable]].
A [=scroll container=] is said to be scrolled to its [=scroll origin=]
when its [=scroll origin=] coincides with the corresponding corner of its [=scrollport=].
This [=scroll position=], the <dfn export>scroll origin position</dfn>,
usually, but not always,
coincides with the [=initial scroll position=].
<div class="example">
For example,
the 'align-content' and 'justify-content' properties [[CSS-ALIGN-3]]
and [=scroll snapping=] [[CSS-SCROLL-SNAP-1]]
can change the [=initial scroll position=]
away from the [=scroll origin position=].
</div>
ISSUE: Check whether things like [=baseline alignment=]
depend on the [=initial scroll position=] or the [=scroll origin position=].
ISSUE: This doesn't define a coordinate system for [=scroll offsets=].
Whether they increase downward/rightward,
block/inline-axis endward,
or away from the [=scroll origin=] is not defined.
Should each API define its coordinate model?
The root viewport, which scrolls the page [=canvas=],
uses the <a>principal writing mode</a> for determining
its [=scroll origin=] and [=initial scroll position=].
Note: In the case where a [=scroll container=] (or one of its ancestors)
is the target of a graphical transform,
the UA might need to take this transform into account
when mapping user inputs to scrolling operations.
For instance, on a touch screen where the user scrolls
by directly dragging the content,
the transform would be expected to be taken into account
to match the direction of scrolling to the gesture.
On the other hand, other user inputs
(such as the Page Down key, or a 1D scroll wheel)
might be more naturally interpreted ignoring the transform.
Choosing the appropriate behavior for each scrolling mechanism
is the responsibility of the UA.
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<h2 id="overflow-properties">
Scrolling and Clipping Overflow</h2>
<h3 id="overflow-control">
Managing Overflow: the 'overflow-x', 'overflow-y', and 'overflow' properties</h3>
These properties specify whether a box’s [=overflow=] is clipped,
and if so,
whether it is a [=scroll container=].
<pre class=propdef>
Name: overflow-x, overflow-y, overflow-block, overflow-inline
Value: visible | hidden | clip | scroll | auto
Initial: ''visible''
Applies to: block containers [[!CSS2]], flex containers [[!CSS3-FLEXBOX]], grid containers [[!CSS3-GRID-LAYOUT]]
Inherited: no
Logical property group: overflow
Percentages: N/A
Computed value: usually specified value, but see text
Animation type: discrete
</pre>
The 'overflow-x' property specifies
the handling of <a>overflow</a> in the horizontal axis
(i.e., overflow from the left and right sides of the box),
and the 'overflow-y' property specifies the handling
of <a>overflow</a> in the vertical axis
(i.e., overflow from the top and bottom sides of the box).
The 'overflow-block' and 'overflow-inline' properties likewise
specify the handling of [=overflow=]
in the [=block axis|block=] and [=inline axis|inline=] axis, respectively
These four properties form a [=logical property group=]
together with the 'overflow' [=shorthand=],
and interact as defined in [[css-logical-1#box]].
<pre class="propdef shorthand">
Name: overflow
Value: <<'overflow-block'>>{1,2}
Initial: visible
Applies to: block containers [[!CSS2]], flex containers [[!CSS3-FLEXBOX]], and grid containers [[!CSS3-GRID-LAYOUT]]
Inherited: no
Percentages: N/A
Computed value: see individual properties
Animation type: discrete
</pre>
The 'overflow' property is a [=shorthand property=]
that sets the specified values of 'overflow-x' and 'overflow-y' in that order.
If the second value is omitted, it is copied from the first.
Values have the following meanings:
<dl dfn-for="overflow, overflow-x, overflow-y" dfn-type="value">
<dt><dfn>visible</dfn>
<dd>
There is no special handling of overflow, that is,
the box’s content is rendered outside the box if positioned there.
The box is not a <a>scroll container</a>.
<dt><dfn>hidden</dfn>
<dd>
This value indicates that
the box’s content is clipped to its [=padding box=]
and that the UA must not provide any scrolling user interface
to view the content outside the clipping region,
nor allow scrolling by direct intervention of the user,
such as dragging on a touch screen
or using the scrolling wheel on a mouse.
However, the content must still be scrollable programmatically,
for example using the mechanisms defined in [[CSSOM-VIEW]],
and the box is therefore still a <a>scroll container</a>.
<dt><dfn>clip</dfn>
<dd>
This value indicates that
the box’s content is clipped to its [=overflow clip edge=]
and that no scrolling user interface should be provided by the UA
to view the content outside the clipping region.
In addition, unlike ''overflow: hidden''
which still allows programmatic scrolling,
''overflow: clip'' forbids scrolling entirely,
through any mechanism,
and therefore the box is not a <a>scroll container</a>.
Unlike ''hidden'', this value <strong>does not</strong> cause
the element to establish a new formatting context.
Note: Authors who also want the box to establish a formatting context
may use ''display: flow-root'' together with ''overflow: clip''.
<dt><dfn>scroll</dfn>
<dd>
This value indicates that
the content is clipped to the [=padding box=],
but can be scrolled into view
(and therefore the box is a <a>scroll container</a>).
Furthermore, if the user agent uses a scrolling mechanism
that is visible on the screen (such as a scroll bar or a panner),
that mechanism should be displayed
whether or not any of its content is clipped.
This avoids any problem with scrollbars appearing
and disappearing in a dynamic environment.
When the target medium is ''@media/print'',
overflowing content may be printed;
it is not defined where it may be printed.
<dt><dfn>auto</dfn>
<dd>
Like ''overflow/scroll'' when the box has <a>scrollable overflow</a>;
like ''overflow/hidden'' otherwise.
Thus, if the user agent uses a scrolling mechanism
that is visible on the screen (such as a scroll bar or a panner),
that mechanism will only be displayed
if there is overflow.
</dl>
The ''visible''/''overflow/clip'' values of 'overflow'
compute to ''overflow/auto''/''hidden'' (respectively)
if one of 'overflow-x' or 'overflow-y' is neither ''visible'' nor ''overflow/clip''.
If the computed value of 'overflow' on a <a>block box</a>
is neither ''overflow/visible'' nor ''overflow/clip'' nor a combination thereof,
it [=establishes an independent formatting context=] for its contents.
User agents must also support
the <dfn value for="overflow, overflow-x, overflow-y, overflow-block, overflow-inline">overlay</dfn> keyword
as a [=legacy value alias=] of ''overflow/auto''.
<h4 id="scroll-visibility">
Interaction of 'visibility' and 'overflow'</h4>
If the computed value of the 'visibility' property is ''visibility/hidden''
(or ''visibility/collapse'' when it has the same effect as ''visibility/hidden''),
and 'overflow' is either ''overflow/scroll'' or ''overflow/auto'',
then:
* The user agent must not make any scrolling mechanism visible.
To the extent that the scrolling mechanism that would normally be visible
in the absence of ''visibility: hidden'' affects layout,
it continues to do so, but is not painted.
* As would be the case with ''overflow: hidden'',
scrolling directly triggered by user interactions is disabled,
but programmatic scrolling continues to take effect.
* The lack of interactive direct scrolling is enforced
even if the user interacts (e.g. with a mouse scrolling wheel)
with a descendent of the ''visibility: hidden'' [=scroll container=]
that is itself set to ''visibility: visible''.
<h4 id="corner-clipping">
Interaction of 'border-radius' and 'overflow'</h4>
As mentioned in [[css-backgrounds-3#corner-clipping]],
the clipping region established by 'overflow' can be rounded:
* When 'overflow-x' and 'overflow-y' compute to
''overflow/hidden'', ''overflow/scroll'', or ''overflow/auto'',
the clipping region is rounded
based on the border radius,
adjusted to the [=padding edge=],
as described in [[css-backgrounds-3#corner-shaping]].
* When both 'overflow-x' and 'overflow-y' compute to ''overflow/clip'',
the clipping region is rounded as described in [[#overflow-clip-margin]].
* However, when one of 'overflow-x' or 'overflow-y' computes to ''overflow/clip''
and the other computes to ''overflow/visible'',
the clipping region is not rounded.
<h4 id="static-media">
Overflow in Print and Other Static Media</h4>
<p class="advisement">
Since scrolling is not possible in static media
(such as print)
authors should be careful to make content accessible in such media,
for example by using ''@media print, (update: none) { … }''
to adjust layout such that all relevant content
is simultaneously visible.
On <a>scroll containers</a> in non-interactive media
with an 'overflow' value of ''overflow/auto'' or ''overflow/scroll''
(but not ''overflow/hidden'')
UAs may display an indication of any scrollable overflow,
such as by displaying scrollbars
or an ellipsis.
Note: Not all [=paged media=] is non-interactive:
for example, e-book readers paginate content,
but are interactive.
<h3 id="overflow-clip-margin">
Expanding Clipping Bounds: the 'overflow-clip-margin' property</h3>
<pre class=propdef>
Name: overflow-clip-margin
Value: <<visual-box>> || <<length [0,∞]>>
Initial: ''0px''
Inherited: no
Applies to: boxes to which 'overflow' applies
Computed value: the computed <<length>> and a <<visual-box>> keyword
Animation type: per computed value if the <<visual-box>> values match; otherwise discrete
</pre>
This property defines the <dfn export>overflow clip edge</dfn> of the box,
i.e. precisely <em>how far</em> outside its bounds
the box’s content is allowed to paint
before being clipped
by effects (such as ''overflow: clip'', above)
that are defined to clip to the box’s [=overflow clip edge=].
Values are defined as follows:
<dl dfn-type=value dfn-for=overflow-clip-margin>
: <dfn><<visual-box>></dfn>
::
Specifies the box edge to use as the [=overflow clip edge=] origin,
i.e. when the specified offset is zero.
If omitted,
defaults to ''overflow-clip-margin/padding-box''.
: <dfn><<length [0,∞]>></dfn>
::
The specified offset dictates
how much the [=overflow clip edge=] is expanded from
the specified box edge
Negative values are invalid.
Defaults to zero if omitted.
</dl>
The [=overflow clip edge=] is shaped in the corners
exactly the same way as an <a spec=css-backgrounds-3>outer box-shadow</a>
with a spread radius of the same cumulative offset
from the box’s [=border edge=].
See [[css-backgrounds-3#corner-shaping]]
and [[css-backgrounds-3#shadow-shape]],
noting in particular the formula for outsets beyond the [=border edge=].
Note: This property has no effect on boxes
with ''overflow: hidden'' or ''overflow: scroll'',
which are not defined to use the [=overflow clip edge=].
<h3 id="overflow-propagation">
Overflow Viewport Propagation</h3>
UAs must apply the 'overflow-*' values
set on the root element to the <a>viewport</a>
when the root element’s 'display' value is not ''display/none''.
However,
when the root element is an [[!HTML]] <{html}> element
(including <a href="https://html.spec.whatwg.org/multipage/introduction.html#html-vs-xhtml">XML syntax for HTML</a>)
whose 'overflow' value is ''overflow/visible'' (in both axes),
and that element has as a child
a <{body}> element whose 'display' value is also not ''display/none'',
user agents must instead apply the 'overflow-*' values
of the first such child element to the viewport.
The element from which the value is propagated must then have
a used 'overflow' value of ''overflow/visible''.
Note: Using [=containment=] on the HTML <{html}> or <{body}> elements disables
this special handling of the HTML <{body}> element.
See the [[CSS-CONTAIN-1#contain-property]] for details.
Note: ''overflow: hidden'' on the root element
might not clip everything outside the [=Initial Containing Block=]
if the ICB is smaller than the viewport,
which can happen on mobile.
If ''overflow/visible'' is applied to the viewport,
it must be interpreted as ''overflow/auto''.
If ''overflow/clip'' is applied to the viewport,
it must be interpreted as ''overflow/hidden''.
<h3 id=smooth-scrolling caniuse=css-scroll-behavior>
Smooth Scrolling: the 'scroll-behavior' Property</h3>
<pre class=propdef>
Name: scroll-behavior
Value: auto | smooth
Initial: auto
Applies to: [=scroll containers=]
Inherited: no
Computed value: specified value
Animation Type: not animatable
Canonical Order: per grammar
</pre>
The 'scroll-behavior' property specifies the scrolling behavior for a [=scroll container=],
when scrolling happens due to navigation,
scrolling APIs [[!CSSOM-VIEW]],
or scroll snapping operations not initiated by the user [[!CSS-SCROLL-SNAP-1]].
Any other scrolls, e.g. those that are performed by the user, are not affected by this property.
When this property is specified on the root element, it applies to the <a>viewport</a> instead.
Note: The 'scroll-behavior' property of the HTML <{body}> element is <em>not</em> propagated to the viewport.
<dl dfn-type=value dfn-for=scroll-behavior>
<dt><dfn>auto</dfn>
<dd>
The [=scroll container=] is scrolled in an <a spec=cssom-view lt="instant scroll">instant</a> fashion.
<dt><dfn>smooth</dfn>
<dd>
The [=scroll container=] is scrolled in a <a spec=cssom-view lt="smooth scroll">smooth</a> fashion
using a user-agent-defined timing function over a user-agent-defined period of time.
User agents should follow platform conventions, if any. <!--fingerprint-->
</dl>
User agents may ignore this property. <!--fingerprint-->
<h2 id="scrollbar-layout">
Scrollbars and Layout</h2>
<h3 id="scrollbar-sizing">
Scrollbar Contributions to Sizing</h3>
When reserving space for a scrollbar placed at the edge of an element's box,
the reserved space is inserted between the inner border edge
and the outer padding edge.
For the purpose of the <a spec=css-backgrounds>background positioning area</a> and <a spec=css-backgrounds>background painting area</a> however,
this reserved space is considered to be part of the [=padding box=].
<div class="example">
In the following document fragment,
both an absolutely-positioned element
and a background image
are positioned to the top right of the box.
<xmp highlight=html>
<style>
article {
background: top right no-repeat url(circle.png);
position: relative;
overflow: auto; }
aside { position: absolute; top: 0; right: 0; }
</style>
<article>
<aside>×</aside>
</article>
</xmp>
If no scrollbars are present on <code><article></code>,
they will both coincide in the top right padding edge corner.
However, if scrollbars are present
then <code><aside></code> will be completely visible,
on the right padding-box edge adjacent to the scrollbars;
whereas the background image will be tucked underneath the scrollbars,
in the top right corner of the scrollbar-extended <a spec=css-backgrounds>background positioning area</a>.
</div>
When the box is intrinsically sized,
this reserved space is added to the size of its contents.
It is otherwise subtracted from space allotted to the [=content area=].
To the extent that the presence of scrollbars can affect sizing,
UAs must start with the assumption that no scrollbars are needed,
and recalculate sizes if it turns out they are.
<div class="example">
In the following document fragment,
the outer <code><article></code> has ''height: auto'', but ''max-height: 5em''.
The inner <code><section></code> has large margins and would normally just fit:
<xmp highlight=html>
...
article { overflow: auto; max-height: 5em; width: max-content; }
section { margin: 2em; line-height: 1 }
...
<article>
<section>
This section has big margins.
</section>
</article>
</xmp>
If we assumed that <code><article></code> needed scrollbars,
then the height of <code><section></code>,
including the single line of text and twice 2em of margins,
adds up to 5em plus a scrollbar.
Since that is greater than 5em, the maximum allowed height,
it seems we made the right assumption and d1 indeed needs scrollbars.
However, we should have started by assuming that no scrollbars are needed.
In that case the content height of <code><article></code>
is exactly the maximum height of 5em,
proving that the assumption was correct
and <code><article></code> indeed should not have scrollbars.
</div>
<h3 id="scrollbar-gutter-property">
Reserving space for the scrollbar: the 'scrollbar-gutter' property</h3>
The space between the inner border edge and the outer padding edge
which user agents may reserve to display the scrollbar
is called the <dfn export>scrollbar gutter</dfn>.
The 'scrollbar-gutter' property gives control to the author
over the presence of <a>scrollbar gutters</a>
separately from the ability to control the presence of scrollbars
provided by the 'overflow' property.
<pre class="propdef">
Name: scrollbar-gutter
Value: auto | stable && both-edges?
Initial: auto
Applies to: [=scroll containers=]
Inherited: no
Computed value: specified keyword(s)
Animation type: discrete
</pre>
This property affects the presence of <a>scrollbar gutters</a>
placed at the <a>inline start</a> edge or <a>inline end</a> edge of the box.
The presence of a <a>scrollbar gutter</a>
at the <a>block start</a> edge and <a>block end</a> edge of the box
cannot be controlled in this level,
and is determined the same way as the presence of <a>scrollbar gutters</a>
placed at the <a>inline start</a> edge or <a>inline end</a> edge of the box
when 'scrollbar-gutter' is ''scrollbar-gutter/auto''.
Scrollbars which by default are placed over the content box
and do not cause <a>scrollbar gutters</a> to be created
are called <dfn export>overlay scrollbars</dfn>.
Such scrollbars are usually partially transparent, revealing the content behind them if any.
Their appearance and size may vary
based on whether and how the user is interacting with them.
Scrollbars which are always placed in a <a>scrollbar gutter</a>,
consuming space when present,
are called <dfn export>classic scrollbars</dfn>.
Such scrollbars are usually opaque.
Whether <a>classic scrollbars</a> or <a>overlay scrollbars</a> are used,
the appearance and size of the scrollbar,
and whether scrollbars appear on the start or end edge of the box,
is UA defined.
Note: Which side a scrollbar appears on may depend on
operating system conventions,
[=bidirectionality=],
or other ergonomic considerations.
In the case of <a>classic scrollbars</a>,
the width of the <a>scrollbar gutter</a>,
if present (see below),
is the same as the width of the scrollbar.
In the case of <a>overlay scrollbars</a>,
no <a>scrollbar gutter</a> is present.
30CB
Note: There are known use cases that could be addressed
by enabling <a>scrollbar gutters</a>
for <a>overlay scrollbars</a>,
but no satisfactory design has been agreed to so far.
This could be addressed by future extensions of this property.
See [[css-overflow-4#sbg-ext]].
The values of this property have the following meaning:
<dl dfn-for="scrollbar-gutter" dfn-type=value>
<dt><dfn>auto</dfn>
<dd><a>Classic scrollbars</a> consume space by creating a <a>scrollbar gutter</a>
when 'overflow' is ''overflow/scroll'',
or when 'overflow' is ''overflow/auto'' and the box is overflowing.
<a>Overlay scrollbars</a> do not consume space.
<dt><dfn>stable</dfn>
<dd>The <a>scrollbar gutter</a> is present
for <a>classic scrollbars</a>
when 'overflow' is ''overflow/hidden'', ''overflow/scroll'', or ''overflow/auto'',
regardless of whether the box is actually overflowing.
<a>Overlay scrollbars</a> do not consume space.
Note: This does not change whether the scrollbar itself is visible,
only the presence of a gutter is affected.
<dt><dfn>both-edges</dfn>
<dd>If a <a>scrollbar gutter</a> would be present
on one of the inline start edge or the inline end edge of the box,
another <a>scrollbar gutter</a> must be present on the opposite edge as well.
</dl>
When the <a>scrollbar gutter</a> is present but the scrollbar is not,
or the scrollbar is transparent or otherwise does not fully obscure the <a>scrollbar gutter</a>,
the background of the <a>scrollbar gutter</a> must be painted as an extension of the padding.
As for the 'overflow' property,
when 'scrollbar-gutter' is set on the root element,
the user agent must apply it to the viewport instead,
and the used value on the root element itself is ''scrollbar-gutter: auto''.
However, unlike the 'overflow' property,
the user agent must not propagate 'scrollbar-gutter' from the HTML <{body}> element.
<div class=note>
Note: The following table summarizes the interaction of 'overflow' and 'scrollbar-gutter'
showing in which case space is reserved for a <a>classic scrollbar</a>'s <a>scrollbar gutter</a>.
<style>
.data.complex tbody th:last-of-type {
border-right: solid black 2px;
}
.data.complex tbody tr:nth-of-type(2n) td,
.data.complex tbody tr:nth-of-type(2n) th:not([rowspan]) {
background: rgba(127, 127, 127, 0.25);
}
</style>
<table class="data complex">
<caption>
Should a <a>classic scrollbar</a>'s <a>scrollbar gutter</a> be present?</caption>
<thead>
<tr>
<th>'overflow'
<th>'scrollbar-gutter'
<th>Overflowing
<th>Not overflowing
</thead>
<tr>
<th rowspan=2>''overflow/scroll''
<th>''scrollbar-gutter/auto''
<td>yes
<td>yes
<tr>
<th>''scrollbar-gutter/stable''
<td>yes
<td>yes
<tr>
<th rowspan=2>''overflow/auto''
<th>''scrollbar-gutter/auto''
<td>yes
<td>
<tr>
<th>''scrollbar-gutter/stable''
<td>yes
<td>yes
<tr>
<th rowspan=2>''overflow/hidden''
<th>''scrollbar-gutter/auto''
<td>
<td>
<tr>
<th>''scrollbar-gutter/stable''
<td>yes
<td>yes
<tr>
<th rowspan=2>''overflow/visible'', ''overflow/clip''
<th>''scrollbar-gutter/auto''
<td>
<td>
<tr>
<th>''scrollbar-gutter/stable''
<td>
<td>
</table>
</div>
<h2 id="auto-ellipsis">
Automatic Ellipses</h2>
<h3 id="text-overflow" caniuse="text-overflow">
Overflow Ellipsis: the 'text-overflow' property</h3>
<pre class="propdef">
Name: text-overflow
Value: clip | ellipsis
Initial: clip
Applies to: block containers
Inherited: no
Percentages: N/A
Computed value: specified keyword
Animation type: discrete
</pre>
This property specifies rendering when inline content overflows
its <a>end</a> line box edge
in the inline progression direction of its block container element ("the block")
that has 'overflow'
other than ''visible''.
Text can overflow for example when it is prevented from wrapping
(e.g. due to <code class="lang-css">white-space: nowrap</code>
or a single word is too long to fit).
Values have the following meanings:
<dl data-dfn-for="text-overflow" class="valuedef">
<dt><dfn id=overflow-clip dfn-type=value>clip</dfn>
<dd>