Title: CSS Overflow Module Level 3
Status: ED
Work Status: Revising
ED: https://drafts.csswg.org/css-overflow-3/
Shortname: css-overflow
Group: csswg
Level: 3
TR: https://www.w3.org/TR/css-overflow-3/
Previous version: https://www.w3.org/TR/2021/WD-css-overflow-3-20211223/
Former Editor: L. David Baron, Mozilla https://www.mozilla.org/, https://dbaron.org/, w3cid 15393
Editor: Elika J. Etemad / fantasai, Invited Expert, http://fantasai.inkedblade.net/contact, w3cid 35400
Editor: Florian Rivoal, On behalf of Bloomberg, http://florian.rivoal.net/, w3cid 43241
Abstract: This module contains the features of CSS relating to scrollable overflow handling in visual media.
This level is focused on completing a precise specification for the existing overflow features,
including the 'overflow' property and its longhands;
'line-clamp', its longhands, and its legacy pre-standard syntax;
and the 'text-overflow' property.
A few additional features introduced in support of [[CSS-CONTAIN-1]] and [[CSS-CONTAIN-2]] are also defined:
''overflow: clip'' and the 'overflow-clip-margin' property.
Ignored Vars: B, P
Ignored Terms:viewport, flex order, -webkit-box-orient
At Risk: the 'max-lines' property
At Risk: application of 'overflow' to [=replaced elements=]
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 deferred until Level 4.
Work will resume on fragmented overflow once this level stabilizes completed.
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.
Value Definitions
This specification follows the CSS property definition conventions from [[!CSS2]]
using the value definition syntax from [[!CSS-VALUES-3]].
Value types not defined in this specification are defined in CSS Values & Units [[!CSS-VALUES-3]].
Combination with other CSS modules may expand the definitions of these value types.
In addition to the property-specific values listed in their definitions,
all properties defined in this specification
also accept the CSS-wide keywords as their property value.
For readability they have not been repeated explicitly.
CSS uses the term overflow to describe
the contents of a box
that extend outside one of that box's edges
(i.e., its content edge, padding edge,
border edge, or margin edge).
The term might be interpreted as
elements or features that cause this overflow,
the non-rectangular region occupied by these features,
or, more commonly, as
the minimal rectangle that bounds that region.
A box's overflow is computed based on the layout and styling of the box itself
and of all descendants whose containing block chain
includes the box.
In most cases, overflow
can be computed for any box
from the bounds and properties of that box itself,
plus the overflow
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:
The ink overflow 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 scrollable overflow area,
such as box shadows,
border images,
text decoration,
overhanging glyphs (with negative side bearings,
or with ascenders/descenders extending outside the em box),
outlines,
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 ink overflow is undefined.
The ink overflow area is the non-rectangular area
occupied by the ink overflow
of a box and its contents,
and the ink overflow rectangle is
the minimal rectangle whose axes are aligned to the box's axes
and that contains the ink overflow area.
Note that the ink overflow rectangle 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=]).
Scrollable Overflow
The scrollable overflow 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 scrollable overflow area is the non-rectangular region
occupied by the scrollable overflow, and the
scrollable overflow rectangle is
the minimal rectangle whose axes are aligned to the box's axes
and that contains the scrollable overflow area.
The scrollable overflow area is the union of:
The [=scroll container=]’s own [=padding box=].
All [=line boxes=] directly contained by the [=scroll container=].
The border boxes
of all boxes for which it is the containing block
and whose border boxes are positioned not wholly outside
its block-start or inline-start padding edges,
accounting for transforms by projecting each box onto
the plane of the element that establishes its 3D rendering context.
[[!CSS3-TRANSFORMS]]
Issue: Is this description of handling transforms
sufficiently accurate?
Border boxes with zero area
do not affect the [=scrollable overflow area=].
The margin areas of grid item and flex item boxes
for which the box establishes a containing block.
The UA may additionally 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.
This needs further testing and investigation;
is therefore deferred in this draft.
The scrollable overflow areas 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 scrollable overflow is not already clipped
(e.g. by the 'clip' property or the 'contain' property).
Issue: should ''overflow: clip'' also clip the scrollable overflow
or should it remain a pure paint-time operation,
which would mean that scrollable overflow, while invisible, would still be scrollable.
Note: The 'mask-*' properties [[!CSS-MASKING-1]]
do not affect the scrollable overflow area.
Issue: Need to evaluate compat of honoring or ignoring 'clip' and 'clip-path'.
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.
Issue: Replace this image with a proper SVG.
Note: The scrollable overflow rectangle 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.
Scrolling and Clipping Overflow: the 'overflow-x', 'overflow-y', and 'overflow' properties
These properties specify whether a box’s content
(including any ink overflow)
is clipped to its padding edge,
and if so,
whether it is a scroll container
that allows the user to scroll clipped parts of its scrollable overflow area
into view.
The visual viewport of the scroll container
(through which the scrollable overflow area can be viewed)
coincides with its padding box,
and is called the scrollport.
For convenience, a box’s nearest scrollport
is the [=scrollport=] of its nearest [=scroll container=] ancestor.
Name: overflow-x, overflow-y
Value: visible | hidden | clip | scroll | auto
Initial: ''visible''
Applies to: block containers [[!CSS2]], flex containers [[!CSS3-FLEXBOX]], grid containers [[!CSS3-GRID-LAYOUT]], and [=replaced=] elements
Inherited: no
Percentages: N/A
Computed value: usually specified value, but see text
Animation type: discrete
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).
Name: overflow
Value: [ visible | hidden | clip | scroll | auto ]{1,2}
Initial: visible
Applies to: block containers [[!CSS2]], flex containers [[!CSS3-FLEXBOX]], and grid containers [[!CSS3-GRID-LAYOUT]]
Inherited: no
Percentages: N/A
Computed value: see individual properties
Animation type: discrete
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:
visible
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 scroll container.
hidden
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 scroll container.
clip
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 scroll container.
Unlike ''hidden'', this value does not 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''.
scroll
This value indicates that
the content is clipped to the padding box,
but can be scrolled into view
(and therefore the box is a scroll container).
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.
auto
Like ''overflow/scroll'' when the box has scrollable overflow;
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.
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 block box
is neither ''overflow/visible'' nor ''overflow/clip'' nor a combination thereof,
it [=establishes an independent formatting context=] for its contents.
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''.
Note: In the case where the [=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.
Overflow in Print and Other Static Media
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 scroll containers 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 page content,
but are interactive.
Scrollbars and Layout
When reserving space for a scrollar placed at the edge of an element's box,
the reserved space is inserted between the inner border edge
and the outer padding edge.
For the purpose of the background positioning area and background painting area however,
this reserved space is considered to be part of the [=padding box=].
In the following document fragment,
both an absolutely-positioned element
and a background image
are positioned to the top right of the box.
<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>
If no scrollbars are present on <article>,
they will both coincide in the top right padding edge corner.
However, if scrollbars are present
then <aside> 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 background positioning area.
When the box is intrinsically sized,
this reserved space is added to the size of its contents.
It is otherwise subtracted from space alotted to the [=content area=].
To the extent that the presence of scrollbars can affect sizing,
UAs must start with the assumption that no scrollbars are needed,
and recalculate sizes if it turns out they are.
In the following document fragment,
the outer <article> has ''height: auto'', but ''max-height: 5em''.
The inner <section> has large margins and would normally just fit:
...
article { overflow: auto; max-height: 5em; width: max-content; }
section { margin: 2em; line-height: 1 }
...
<article>
<section>
This section has big margins.
</section>
</article>
If we assumed that <article> needed scrollbars,
then the height of <section>,
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 <article>
is exactly the maximum height of 5em,
proving that the assumption was correct
and <article> indeed should not have scrollbars.
Scrolling Origin, Direction, and Restriction
The initial scroll position is
the initial position of
the box’s scrollable overflow area
with respect to its border box,
prior to any user or programmatic scrolling that changes it.
The initial scroll position
is dependent on the box’s writing mode,
and is by default the block-start/inline-start edge
of the box’s padding edge.
However, the 'align-content' and 'justify-content' properties [[!CSS-ALIGN-3]]
can be used to change this,
see [[css-align-3#overflow-scroll-position]].
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 the scrollable overflow area
of scroll containers
on the block-start and inline-start sides of the box
(thereby behaving as if they had no scrollable overflow on that side).
The viewport uses the principal writing mode for these calculations.
Expanding Clipping Bounds: the 'overflow-clip-margin' property
Name: overflow-clip-margin
Value: <> || <>
Initial: ''0px''
Inherited: no
Applies to: boxes to which 'overflow' applies
Computed value: the computed <> and a <> keyword
Animation type: per computed value if the <> values match; otherwise discrete
This property defines the overflow clip edge of the box,
i.e. precisely how far outside its bounds
the box’s content is allowed to paint
before being clipped
by effects (such as ''overflow: clip'', above)
that are defined to clip to the box’s [=overflow clip edge=].
Values are defined as follows:
: <>
::
Specifies the box edge to use as the [=overflow clip edge=] origin,
i.e. when the specified offset is zero.
If omitted,
defaults to ''overflow-clip-margin/padding-box''.
: <>
::
The specified offset dictates
how much the [=overflow clip edge=] is expanded from
the specified box edge
Negative values are invalid.
Defaults to zero if omitted.
The [=overflow clip edge=] is shaped in the corners
exactly the same way as an outer box-shadow
with a spread radius of the same cumulative offset
from the box’s [=border edge=].
See [[css-backgrounds-3#corner-shaping]]
and [[css-backgrounds-3#shadow-shape]],
noting in particular the formula for outsets beyond the [=border edge=].
Note: This property has no effect on boxes
with ''overflow: hidden'' or ''overflow: scroll'',
which are not defined to use the [=overflow clip edge=].
Overflow Viewport Propagation
UAs must apply the 'overflow-*' values
set on the root element to the viewport
when the root element’s 'display' value is not ''display/none''.
However,
when the root element is an [[!HTML]] <{html}> element
(including XML syntax for HTML)
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''.
Flow Relative Properties: the 'overflow-block' and 'overflow-inline' properties
The following properties are processed according to the same processing model
as the Flow-Relative box model properties defined in [[css-logical-1#box]].
Name: overflow-inline, overflow-block
Value: <<'overflow'>>
Initial: visible
Applies to: Same as 'overflow-x' and 'overflow-y'
Inherited: no
Percentages: N/A
Computed value: Same as 'overflow-x' and 'overflow-y'
Animation type: discrete
These properties correspond to the
'overflow-x' and 'overflow-y' properties.
The mapping depends on the element's 'writing-mode'.
Smooth Scrolling: The 'scroll-behavior' Property
Name: scroll-behavior
Value: auto | smooth
Initial: auto
Applies to: [=scroll containers=]
Inherited: no
Computed value: specified value
Animatable: no
Canonical Order: per grammar
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 viewport instead.
Note: The 'scroll-behavior' property of the HTML <{body}> element is not propagated to the viewport.
auto
The [=scroll container=] is scrolled in an instant fashion.
smooth
The [=scroll container=] is scrolled in a smooth fashion
using a user-agent-defined timing function over a user-agent-defined period of time.
User agents should follow platform conventions, if any.
User agents may ignore this property.
Reserving space for the scrollbar: the 'scrollbar-gutter' property
The space between the inner border edge and the outer padding edge
which user agents may reserve to display the scrollbar
is called the scrollbar gutter.
The 'scrollbar-gutter' property gives control to the author
over the presence of scrollbar gutters
separately from the ability to control the presence of scrollbars
provided by the 'overflow' property.
Name: scrollbar-gutter
Value: auto | stable && both-edges?
Initial: auto
Applies to: [=scroll containers=]
Inherited: no
Computed value: specified keyword(s)
Animation type: discrete
This property affects the presence of scrollbar gutters
placed at the inline start edge or inline end edge of the box.
The presence of a scrollbar gutter
at the block start edge and block end edge of the box
cannot be controlled in this level,
and is determined the same way as the presence of scrollbar gutters
placed at the inline start edge or inline end 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 scrollbar gutters to be created
are called overlay scrollbars.
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 scrollbar gutter,
consuming space when present,
are called classic scrollbars.
Such scrollbars are usually opaque.
Whether classic scrollbars or overlay scrollbars 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 classic scrollbars,
the width of the scrollbar gutter,
if present (see below),
is the same as the width of the scrollbar.
In the case of overlay scrollbars,
no scrollbar gutter is present.
Note: There are known use cases that could be addressed
by enabling scrollbar gutters
for overlay scrollbars,
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:
auto
Classic scrollbars consume space by creating a scrollbar gutter
when 'overflow' is ''overflow/scroll'',
or when 'overflow' is ''overflow/auto'' and the box is overflowing.
Overlay scrollbars do not consume space.
stable
The scrollbar gutter is present
for classic scrollbars
when 'overflow' is ''overflow/hidden'', ''overflow/scroll'', or ''overflow/auto'',
regardless of whether the box is actually overflowing.
Overlay scrollbars do not consume space.
Note: This does not change whether the scrollbar itself is visible,
only the presence of a gutter is affected.
both-edges
If a scrollbar gutter would be present
on one of the inline start edge or the inline end edge of the box,
another scrollbar gutter must be present on the opposite edge as well.
When the scrollbar gutter is present but the scrollbar is not,
or the scrollbar is transparent or otherwise does not fully obscure the scrollbar gutter,
the background of the scrollbar gutter 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.
Note: The following table summarizes the interaction of 'overflow' and 'scrollbar-gutter'
showing in which case space is reserved for a classic scrollbar's scrollbar gutter.
This property specifies rendering when inline content overflows
its end 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 white-space: nowrap
or a single word is too long to fit).
Values have the following meanings:
clip
Clip inline content that overflows its block container element. Characters may be only partially rendered.
ellipsis
Render an ellipsis character (U+2026)
to represent clipped inline content.
Implementations may substitute a more language, script, or writing-mode appropriate
ellipsis character,
or three dots "..." if the ellipsis character is unavailable.
The term "character" is used in this property definition
for better readability and means "grapheme cluster" [[!UAX29]]
for implementation purposes.
For the ellipsis value
implementations must hide characters and
atomic inline-level elements
at the end edge of the line as necessary to fit the ellipsis, and
place the ellipsis immediately adjacent
to the end edge of the remaining inline content.
The first character or
atomic inline-level element
on a line
must be clipped rather than ellipsed.
Bidi ellipsis examples
These examples demonstrate which characters get hidden
to make room for the ellipsis in a bidi situation:
those visually at the end edge of the line.
Sample CSS:
Sample HTML fragments, renderings, and your browser:
HTML
Reference rendering
Your Browser
<div>שלום 123456</div>
123456 ם…
שלום 123456
<div dir=rtl>שלום 123456</div>
…456 שלום
שלום 123456
ellipsing details
Ellipsing only affects rendering and must not affect layout
nor dispatching of pointer events:
The UA should dispatch any pointer event on the ellipsis to the elided element,
as if 'text-overflow' had been ''text-overflow/none''.
The ellipsis is styled and baseline-aligned according to
the block.
Ellipsing occurs after relative positioning and other graphical transformations.
If there is insufficient space for the ellipsis,
then clip the rendering of the ellipsis itself
(on the same side that neutral characters on the line
would have otherwise been clipped with the ''text-overflow:clip'' value).
user interaction with ellipsis
When the user is interacting with content
(e.g. editing, selecting, scrolling),
the user agent may treat ''text-overflow: ellipsis'' as ''text-overflow: clip''.
Selecting the ellipsis should select the ellipsed text.
If all of the ellipsed text is selected,
UAs should show selection of the ellipsis.
Behavior of partially-selected ellipsed text is up to the UA.
Example(s):
text-overflow examples
These examples demonstrate setting the text-overflow of a block container element
that has text which overflows its dimensions:
sample CSS for a div:
Note: the side of the line that the ellipsis is placed depends on the 'direction' of the block.
E.g. an overflow hidden right-to-left
(direction: rtl)
block clips inline content on the left side,
thus would place a text-overflow ellipsis on the left
to represent that clipped content.
ellipsis interaction with scrolling interfaces
This section applies to elements with text-overflow other than ''text-overflow:clip''
(non-clip text-overflow)
and overflow:scroll.
When an element with non-clip text-overflow has overflow of scroll
in the inline progression dimension of the text,
and the browser provides a mechanism for scrolling
(e.g. a scrollbar on the element,
or a touch interface to swipe-scroll, etc.),
there are additional implementation details that provide a better user experience:
When an element is scrolled (e.g. by the user, DOM manipulation),
more of the element's content is shown.
The value of text-overflow should not affect
whether more of the element's content is shown or not.
If a non-clip text-overflow is set,
then as more content is scrolled into view,
implementations should show whatever additional content fits,
only truncating content which would otherwise be clipped
(or is necessary to make room for the ellipsis/string),
until the element is scrolled far enough
to display the edge of the content
at which point that content should be displayed
rather than an ellipsis/string.
This example uses text-overflow on an element with overflow scroll
to demonstrate the above described behavior.
sample CSS:
<div class="crawlbar">
CSS is awesome, especially when you can scroll
to see extra text instead of just
having it overlap other text by default.
</div>
demonstration of sample CSS and HTML:
CSS is awesome, especially when you can scroll
to see extra text instead of just
having it overlap other text by default.
While the content is being scrolled, implementations may adjust their rendering of ellipses (e.g. align to the box edge rather than line edge).
Indicating Block-Axis Overflow: the 'block-ellipsis' property
Name: block-ellipsis
Value: none | auto | <>
Initial: none
Applies to: [=block containers=]
Inherited: yes
Percentages: N/A
Computed value: specified value
Animation type: discrete
This property allows inserting content into the last line box
before a (forced or unforced) region break
to indicate the continuity of truncated/interrupted content.
It only affects line boxes contained directly by the block container itself,
but as it inherits, will have an effect on descendants’ line boxes unless overridden.
If the box contains no line box immediately preceding a region break,
then this property has no effect.
Note: See [[#fragmentation]] for a way to generate boxes with such a [=region break=].
ISSUE: Should this apply to other types of fragmentation breaks (e.g. pages, columns)?
The inserted content is called the block overflow ellipsis.
Values have the following meanings:
none
The rendering is unaffected.
auto
Render an ellipsis character (U+2026)--
or a more typographically-appropriate equivalent--
as the block overflow ellipsis
at the end of the affected line box.
UAs should use the conventions of the
content language,
writing system, and
writing mode
to determine the most appropriate ellipsis string.
<>
Render the specified string
as the block overflow ellipsis
at the end of the affected line box.
The UA may truncate this string if it is absurdly long.
When 'block-ellipsis' is not ''block-ellipsis/none'',
the block overflow ellipsis string
is wrapped in an anonymous inline
and placed at the end of the line box
as a direct child of the block container’s root inline box,
reducing the space in the line box
available to the other contents of the line.
This inline is assigned ''unicode-bidi: plaintext''
and ''line-height: 0''
and is placed in the line box after the last
soft wrap opportunity (see [[!CSS-TEXT-3]])
that would still allow the entire block overflow ellipsis to fit on the line.
For this purpose, soft wrap opportunities added by 'overflow-wrap' are ignored.
If this results in the entire contents of the line box being displaced,
the line box is considered to contain a [=strut=], as defined in [[CSS2/visudet#leading]].
Text alignment and justification
occurs after placement,
and measures the inserted block overflow ellipsis
together with the rest of the line’s content.
Note: Setting the [=block overflow ellipsis=]'s 'line-height' to ''0''
makes sure that inserting it cannot cause the line's height to grow,
which could cause further relayouts and potentially cycles.
This is almost equivalent to inserting the [=block overflow ellipsis=]
as a paint-time operation, except that it still participates in alignment and justification.
The downside is that unusually tall / deep glyphs in the [=block overflow ellipsis=]
may overflow.
The [=block overflow ellipsis=] must not be included
in either the ''::first-letter'' nor the ''::first-line'' pseudo-elements.
If there is a subsequent fragmentation container in the [=fragmentation context=]
that would receive subsequent content,
then the content displaced by the block overflow ellipsis
must be pushed to that fragmentation container.
The UA must treat the block overflow ellipsis as an unbreakable string,
If any part of the [=block overflow ellipsis=] overflows,
it is treated as [=scrollable overflow=],
and its rendering is affected by the 'text-overflow' property.
The block overflow ellipsis does not capture events:
pointer events are dispatched to whatever is underneath it.
It also has no effect on the intrinsic size of the box:
its min-content and max-content sizes
are calculated exactly as if 'block-ellipsis' were ''block-ellipsis/none''.
Note: Future specifications may extend this feature,
for example by providing an ''::ellipsis'' pseudo-element
to style the text,
or by allowing the selection of a child element of the block
to use as either an inline-level or block-level indicator
(in which case, it can capture events).
Fragmenting Overflow
Limiting Visible Lines: the 'line-clamp' shorthand property
The 'line-clamp' property is a shorthand
for the 'max-lines', 'block-ellipsis', and 'continue' properties.
Issue: For the time being,
experimental implementations are encouraged
to follow the full behavior defined by this shorthand and its longhands,
but to only expose the shorthand to authors.
This is in order to facilitate further tweaking,
and in particular potential renaming,
of the longhand properties and their values.
It allows limiting the contents of a block container
to the specified number of lines;
remaining content is fragmented away
and neither rendered nor measured.
Optionally, it also allows inserting content into the last line box
to indicate the continuity of truncated/interrupted content.
The values have the following meaning:
none
Sets 'max-lines' to ''max-lines/none'',
'continue' to ''continue/auto'',
and 'block-ellipsis' to ''block-ellipsis/none''.
<>
Sets 'max-lines' to the specified <>,
'continue' to ''discard'',
and the 'block-ellipsis' property to second component of the value
or to ''block-ellipsis/auto'' if omitted.
See the corresponding longhand properties for details
about how this mechanism operates.
In this example, the lead paragraph of each article
is listed in a shortened menu,
truncated to fit within 5 lines
that end with “… (continued on next page)”:
li {
line-clamp: 5 "… (continued on next page)";
}
strong {
display: block;
text-transform: uppercase;
}
<li><a href="cheese-is-milk">
<strong>Cheese is Actually Made of Milk!</strong>
Investigative reporters at the World Wide Web Press Corps
have discovered the secret of cheese.
Tracing through byzantine layers of bureaucracy and shadow corporations,
our crack team of journalists have traced the source of camembert.
</a></li>
Sample rendering:
+---------------------------------------+
| CHEESE IS ACTUALLY MADE OF MILK! |
| Investigative reporters at the World |
| Wide Web Press Corps have discovered |
| the secret of cheese. Tracing through |
| byzantine… (continued on next page) |
+---------------------------------------+
Legacy compatibility
For compatibility with legacy content,
UAs that support 'line-clamp' must also support the '-webkit-line-clamp' property
and the additional ''-webkit-discard'' value for the 'continue' property.
Like 'line-clamp', '-webkit-line-clamp' is a shorthand of 'max-lines', 'continue', and 'block-ellipsis',
except that:
* its syntax is ''line-clamp/none'' | <>
* it sets 'continue' to ''-webkit-discard'' instead of ''discard''
* it unconditionally sets 'block-ellipsis' to ''block-ellipsis/auto''
Additionally, for children (including anonymous children)
of boxes whose 'display' property computes to ''-webkit-box'' or ''-webkit-inline-box'',
the used values of the 'max-lines', 'continue', and 'block-ellipsis' properties
are taken from the computed values of the parent box;
the computed values of these properties on the box itself are ignored.
The -webkit-discard value behaves identically to ''discard'',
except that it only takes effect
if the computed value of the 'display' property on the parent
is ''-webkit-box'' or ''-webkit-inline-box''
and the computed value of the '-webkit-box-orient' property on the parent
is ''-webkit-box-orient/vertical''.
Note: Implementations of the legacy '-webkit-line-clamp' property
have not behaved identically to what is specified here.
The historical behavior is quirky and less robust,
as documented for example in this blog post.
The current design learns from the mistakes of that early experiment,
and is intended to be sufficiently compatible with existing content
that implementations can eventually be changed to follow to the specified behavior.
If further adjustments are found to be necessary,
they will be incorporated to this specification.
In the meanwhile, authors should be aware that there may be discrepancies.
Forcing a Break After a Set Number of Lines: the 'max-lines' property
Name: max-lines
Value: ''none'' | <>
Initial: ''none''
Applies to: block containers which are also [=fragmentation containers=] that capture [=region breaks=]
Inherited: no
Percentages: N/A
Computed value: the keyword ''max-lines/none'' or an integer
Animation type: by computed value type
This property has no effect on boxes that are not [=fragmentation containers=] that capture [=region breaks=].
Otherwise, if the value of 'max-lines' is not none,
a region break
is forced after its Nth
descendant in-flowline box,
where N is the specified value of 'max-lines'.
Only lines boxes in the same Block Formatting Context
are counted:
the contents of descendants that establish independent formatting contexts
are skipped over while counting line boxes.
If fewer than N line boxes exist,
then 'max-lines' introduces no region break.
As ''continue: discard'' does not cause the element to [=establish an independent formatting context=],
lines in a nested element with 'line-clamp' are counted,
as can be seen in the examples below.
<div id=a>
a: line 1<br>
a: line 2<br>
<div id=b>
b: line 1<br>
b: line 2<br>
b: line 3<br>
b: line 4<br>
</div>
a: line 3<br>
a: line 4<br>
</div>
Sample rendering given #a { line-clamp: 5; } #b { line-clamp: 2; }:
a: line 1
a: line 2
b: line 1
b: line 2…
a: line 3…
Note that in the second case,
the maximum of 2 lines set on element ''#b'' does not take effect,
since a forced break is introduced
before the second line of this element.
Note: This implies that 'max-lines' has no effect when applied to [=multi-column containers=],
since any line box they contain are nested into [=independent formatting contexts=].
Only positive integers are accepted.
Zero or negative integers are invalid
and must cause the declaration to be ignored.
Note: The 'widows', 'orphans', and 'break-inside' properties
do not affect the position of the forced region break
introduced by the 'max-lines' property.
Note: Despite the “region break” name, this is not a dependency on [[CSS-REGIONS-1]].
The word “region” is only used as a classifier for forced breaks:
they can be “page breaks” (breaks across pages [[css-page-3]]),
“column breaks” (breaks across multi-column layout columns [[css-multicol-1]]),
or “region breaks” (breaks across any other kind of CSS-induced fragmentation containers).
If an implementation supports neither [[CSS-REGIONS-1]] nor [[CSS-OVERFLOW-4#fragmentation]],
then it will have had no occasion yet to run into that kind of breaks,
and this will be an addition.
However the addition does not involve bringing over any of the [[CSS-REGIONS-1]] functionality.
All that is needed is:
* be able to fragment
* classify these fragmentation containers as “Category 3”
(i.e. not pages nor columns)
for the purpose of forced breaks.
Fragmentation of Overflow: the 'continue' property
Name: continue
Value: auto | discard
Initial: ''continue/auto''
Applies to: [=block containers=] and [=multicol containers=]
Inherited: no
Percentages: N/A
Computed value: specified keyword
Animation type: discrete
The 'continue' property gives authors the ability
to turn a box into a [=fragmentation container=] (see [[!CSS-BREAK-3]])
and to specify that content after the [=fragmentation break=] must be discarded.
Issue: This property is meant to generalize and replace the region-fragment property from [[CSS-REGIONS-1]].
Once it is sufficiently stable in this specification,
region-fragment should be removed from the regions specification in favor of this.
auto
If the box has more content than can fit,
the excess content is handled according to the usual rules.
discard
The box becomes a [=fragmentation container=]
that captures region breaks,
if it is not already.
[[!CSS-BREAK-3]]
Content after the first [=region break=] is not rendered (see below).
(If the box is a [=multi-column container=],
any [=overflow columns=] are also not rendered.)
Note: This [=region break=] might be forced
(e.g. imposed by 'max-lines' or by another mechanism,
such as the 'break-before'/'break-after' properties)
or [=unforced break|unforced=]
(e.g. if the content would otherwise overflow this [=fragmentation container=]
due to its size constraints).
Breaks applying to other [=fragmentation contexts=]
(such as pagination of this box itself)
do not cause any content to be discarded.
Note: This property does not cause the box to [=establish an independent formatting context=].
Given an article with one excessively long overflowing line,
and four more lines than can fit in the [=fragmentation direction=] as in the illustration bellow,
different renderings are possible based on the combination of the 'overflow' and 'continue' property.
''continue: discard''
''continue: auto''
''overflow: visible''
''overflow: hidden''
Content that is “not rendered” due to ''continue: discard''
is discarded, similar to ''display: none'':
* It is not rendered.
* It is also not made available for speech rendering.
* It does not allow user interaction.
* Issue(2970): Make sure effects on OM are well defined
* Issue(2971): What about [=positioned=] elements whose static position is in the discarded content are not rendered?
See also discussions in the Sydney F2F meeting.
However, since intrinsic sizes are calculated across fragmentation containers,
this content is taken into account
for the purpose of finding the box’s
min-content and max-contentinline sizes
(see [[css-break-3#varying-size-boxes]]).
Min-content and max-contentblock sizes
are calculated based on the content
from the start of the [=fragmented flow=]
to the first [=forced break=] if any,
or to the end of the [=fragmented flow=] if there is no forced break.
Note: In the case of parallel fragmentation flows,
content occurring after the fragmentation break in the box tree
could still be rendered,
if it is laid out above the position
representing the end of this fragmentation container.
Appendix A. Privacy and Security Considerations
===============================================
This specification introduces no new privacy or security concerns.
* Require end-edge [=padding=] to be included
in the [=scrollable overflow area=] of [=block containers=];
this was previously optional for [=block containers=] with ''align-content/normal'' [=content distribution=],
pending Web-compatibility investigations.
(Issue 129)
* Extended 'overflow' to apply to [=replaced elements=].
How exactly this works is still still under discussion.
(Issue 7144)
* More precisely define the interaction of '-webkit-line-clamp'
and the various 'line-clamp' longhands. See [[#webkit-line-clamp]].
(Issue 6842)
* Clarified that [=region breaks=] are what trigger discarding of subsequent content
for ''continue: discard'' (and that other types of breaks do not);
and gave examples of what triggers a [=region break=] on such a box.
* Minor clarifications:
* Annotate <> value definitions using [=CSS bracketed range notation=].
* Moved the 'scroll-behavior' property from [[CSSOM-VIEW]] to this specification.
* Adopted the 'scrollbar-gutter' property, previously defined in [[CSS-OVERFLOW-4]].
* Added box-edge keywords to 'overflow-clip-margin'.
(Issue 5801)
* Clarified interaction of 'border-radius' and 'overflow-clip-margin'.
* Added note about user scrolling gestures vs. transforms.
* Tightened up requirements around incorporating margins, padding, and zero-area boxes into the [=scrollable overflow area=].
(Issue 129,
Issue 4791)
* More precisely defined impact of scrollbars on layout.
(Issue 3348)
* Handle various edge cases involving 'overflow' propagation to the viewport.
(Issue 3779,
Issue 5913)
* Miscellaneous small clarifications and fixes.
* Fixed markup errors and typos
* Made various editorial clarifications and fixed inconsistencies
* Renamed block-overflow to 'block-ellipsis'
* Define 'text-overflow' to affect an overflowing [=block overflow ellipsis=]
* Define the behavior of a [=block overflow ellipsis=] that is longer than the line
* Define interaction of 'block-ellipsis' and ''::first-line'' / ''::first-letter''
* Adjust the intrinsic size of elements with ''continue: discard''
* Define how to layout the [=block overflow ellipsis=]
* Define the legacy behavior of '-webkit-line-clamp' for compatibility with existing content
* Define that end padding is included in the scrollable overflow area except for block containers
(which are more tricky to handle, due to web-compat issues)
* Define that Margins of flex and grid items are included in scrollable overflow
* Do not propagate overflow from boxless elements
* Rename 'scrollable overflow region' to 'scrollable overflow area'
* Introduce the 'overflow-clip-margin' property
Thanks especially to the feedback from
Rossen Atanassov,
Bert Bos,
Tantek Çelik,
John Daggett,
Daniel Glazman,
Vincent Hardy,
Håkon Wium Lie,
Peter Linss,
Robert O'Callahan,
Florian Rivoal,
Alan Stearns,
Steve Zilles,
and all the rest of the
www-style community.
