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Showing content from https://www.w3.org/TR/2018/WD-css-overflow-3-20180731/ below:

CSS Overflow Module Level 3

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 [CSS21] introduced the overflow property, which allows authors to have overflow be handled by scrolling, which means it is no longer an authoring error. It also allows authors to specify that overflow is handled by clipping, which makes sense when the author’s intent is that the content not be shown.

This specification introduces the long-standing de-facto overflow-x and overflow-y properties, adds a 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-overflow toghether.

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:

• ink overflow
• scrollable overflow

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 region, 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 region 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 region. 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]

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 region 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 region.

The scrollable overflow region is the union of:

• the box’s own content and padding areas

  There’s disagreement on the scrolling model. 2.1 apparently defined that you scrolled the *content* area; the content would overflow the content-box, and you would union that overflow with the content box to find the scrollable area. In particular, this means that the content would be offset by the start-sides padding, but if it overflowed, it would go right to the edge on the end sides. This is what Firefox and IE do.

  At least some authors (and spec authors) instead have the mental model that the padding box is what’s scrollable, so when you scroll to the end of the overflow, there’s the right/bottom padding. Chrome/WebKit do this for the block axis, at least. They’re somewhat inconsistent for the inline axis; there’s something weird about how they handle lineboxes.

  It seems that the block-axis padding is probably web-compatible to honor. It’s unclear that the inline-axis padding will be. Further experimentation is needed.

• all line boxes directly contained by the box
• 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]

  Is this description of handling transforms sufficiently accurate?

• the scrollable overflow regions of all of the above boxes (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).

  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 region.

  Need to evaluate compat of honoring or ignoring clip and clip-path.

• Optionally, additional padding on the end-edge sides, corresponding to the end-side padding of the scroll container, such that the end edges of its in-flow content coincide with the end edges of its content area when scrolled to the end.

  It’s not yet clear if including the end-side padding in the scrollable layer is Web-compatible, so this clause is under investigation. It appears that Chrome and Safari include such padding in the block axis; and the behavior in the inline axis is not clear. <https://github.com/w3c/csswg-drafts/issues/129>

The UA may additionally include the margin areas of boxes for which it is the containing block. 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.

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.

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 region into view. The visual viewport of the scroll container (through which the scrollable overflow region can be viewed) coincides with its padding box, and is called the scrollport.

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).

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 programatically, for example using the mechanisms defined in [CSSOM-VIEW], and the box is therefore still a scroll container.

clip

Like hidden, this value indicates that the box’s content is clipped to its padding box 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 print, overflowing content may be printed; it is not defined where it may be printed.

auto

Like scroll when the box has scrollable overflow; like 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.

If the computed value of overflow on a block box is neither visible nor clip nor a combination thereof, it establishes an independent formatting context for its contents.

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 auto or scroll (but not 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.

In the case of a scrollbar being placed on an edge of the element’s box, it should be inserted between the inner border edge and the outer padding edge. Any space taken up by the scrollbars should be taken out of (subtracted from the dimensions of) the containing block formed by the element with the scrollbars.

import examples from [CSS3-BOX].

The initial scroll position, that is, the initial position of the box’s scrollable overflow region with respect to its border box, prior to any user or programmatic scrolling that changes it, 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 Box Alignment 3 §5.3 Overflow and Scroll Positions.

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 region 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.

UAs must apply the overflow-* values set on the root element to the viewport. However, when the root element is an [HTML] html element (including XML syntax for HTML) whose overflow value is visible (in both axes), and that element has a body element as a child, 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 visible.

If visible is applied to the viewport, it must be interpreted as auto. If clip is applied to the viewport, it must be interpreted as hidden.

The following properties are processed according to the same processing model as the Flow-Relative box model properties defined in CSS Logical Properties 1 §3 Flow-Relative Box Model Properties.

These properties correspond to the overflow-x and overflow-y properties. The mapping depends on the element’s writing-mode.

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.

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:

div {
  font-family: monospace;
  white-space: pre;
  overflow: hidden;
  width: 9ch;
  text-overflow: ellipsis;
}

Sample HTML fragments, renderings, and your browser:

123456 ם…

שלום 123456

…456 שלום

שלום 123456

• 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 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).

• 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):

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

sample CSS for a div:

div {  font-family:Helvetica,sans-serif; line-height:1.1;
  width:3.1em; padding:.2em; border:solid .1em black; margin:1em 0;
}

sample HTML fragments, renderings, and your browser:

<div>
CSS IS AWESOME, YES
</div>

CSS IS AWESOME, YES

<div style="text-overflow:clip; overflow:hidden">
CSS IS AWESOME, YES
</div>

CSS IS AWESOME, YES

<div style="text-overflow:ellipsis; overflow:hidden">
CSS IS AWESOME, YES
</div>

CSS IS AWESOME, YES

<div style="text-overflow:ellipsis; overflow:hidden">
NESTED
 <p>PARAGRAPH</p>
WON’T ELLIPSE.
</div>

PARAGRAPH

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.

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.

Example(s):

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

sample CSS:

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

sample HTML fragment:

<div class="crawlbar">
CSS is awesome, especially when you can scroll
to see extra text instead of just
having it overlap other text by default.
</div>

demonstration of sample CSS and HTML:

CSS is awesome, especially when you can scroll to see extra text instead of just having it overlap other text by default.

While the content is being scrolled, implementations may adjust their rendering of ellipses (e.g. align to the box edge rather than line edge).

The names of this property and its values are not fully settled, and alternatives have been suggested. For the time being, experiemental implementations are encouraged to follow the full behavior defined by this property, but to only expose it to authors through its line-clamp shorthand. <https://github.com/w3c/csswg-drafts/issues/2561>

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 §5 Fragmenting Overflow for a way to generate boxes with such a region break.

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:

clip

The rendering is unaffected.

ellipsis

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.

<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-overflow is not clip, 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 is placed in the line box after the last soft wrap opportunity [CSS-TEXT-3] that would still allow the entire block overflow ellipsis to fit on the line. (This can result in the entire contents of the line box being replaced.) For this purpose, soft wrap opportunities added by overflow-wrap are ignored. Text alignment and justification occurs after placement, and measures the inserted block overflow ellipsis together with the rest of the line’s content.

Interaction with ::first-letter and ::first-line needs to be defined. <https://github.com/w3c/csswg-drafts/issues/2906>

If there is no next fragmentation container and thus the remainder of the content after the break would be discarded, then the UA may visually replace the contents of the line, as it does for text-overflow. If, however, there is a next fragmentation container that would receive subsequent content, then the content replaced by the block overflow ellipsis must be pushed to the next fragmentation container and the block overflow ellipsis inserted and laid out exactly as if it were part of the in-flow contents of the line. This can result in changes to layout within or impacted by the line. The means of breaking any resulting cycles is up to the UA.

We may want to get rid of the "MAY" in the previous paragraph, and require a single behavior. Finding how to break the cycles probably should be defined as well. <https://github.com/w3c/csswg-drafts/issues/2905>

If the block overflow ellipsis is too long to fit in the line, the result is undefined. (The UA may, for example, treat the block overflow ellipsis as an unbreakable string, or it may lay out the string across more than one line, replacing content in earlier lines as well.)

There is a suggestion to actually define the behavior when the ellipsis is too long to fit the line. <https://github.com/w3c/csswg-drafts/issues/2882>

The block overflow ellipsis does not capture events: pointer events are dispatched to whatever is underneath or otherwise visually replaced by 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-overflow were clip.

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).

The line-clamp property is a shorthand for the max-lines, block-overflow, and continue properties.

For the time being, experiemental 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 none, continue to auto, and block-overflow to clip.

<integer>

Sets max-lines to the specified <integer>, continue to discard, and the block-overflow property to second component of the value or to ellipsis 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 10 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 bureacracy 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)  |
+---------------------------------------+

For compatibility with legacy content, UAs that support line-clamp must also support -webkit-line-clamp as an alias.

It is not obvious that a naive -webkit- alias can work, due to complicated compatibility constraints. Something more complicated might be needed. <https://github.com/w3c/csswg-drafts/issues/2847>

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-flow line box, where N is the specified value of max-lines. Only lines boxes in the same Block Formatting Context are counted: the contents of descendants that establish independent formatting contexts are skipped over while counting line boxes.

If fewer than N line boxes exist, then max-lines introduces no region break.

Note: This implies that max-lines has no effect when applied to multi-column containers, since any line box they contain are nested into independent formatting contexts.

Only positive integers are accepted. Zero or negative integers are invalid and must cause the declaration to be ignored.

Note: The widows, orphans, and break-inside properties do not affect the position of the forced region break introduced by the max-lines property.

Note: Despite the “region break” name, this is not a dependency on

. The word “region” is only used as a classifier for forced breaks: they can be “page breaks” (break pages), “column breaks” (breaks multicol), or “region breaks” (break any other kind of css-induced fragmentainers).

If an implementation supports neither [CSS-REGIONS-1] nor CSS Overflow Module Level 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 "category3" (i.e. not pages nor multicol) for the purpose of forced breaks.

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.

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]

The position of the first forced region break (whether imposed by max-lines or by another mechanism, such as the break-before/break-after properties) or the position of the first unforced break, which ever comes sooner, is treated as the end of the fragmentation container, and the rest of the fragmented flow is laid out in the next fragmentation container in the fragmentation context. The remaining content after the fragmentation break is discarded, similar to if it were display: none:

• Just as the content is visually hidden, it is not made available for speech rendering

• It does not allow user interaction

• Make sure effects on OM are well defined <https://github.com/w3c/csswg-drafts/issues/2970>

• Positioned elements whose static position is in the discarded content are not rendered.

  Are we sure about that? Discussions in the Syndey F2F meeting seemed to generally converge on this, but other possibilities were raised. <https://github.com/w3c/csswg-drafts/issues/2971>

• It is neither rendered, nor measured as part of the box’s contents for the purpose of finding its min-content, max-content, or automatic block sizes

However, since intrinsic sizes are calculated across all fragmentation containers, this content is taken into account for the purpose of finding the box’s min-content and max-content inline sizes (see CSS Fragmentation 3 §5.1 Breaking into Varying-size Fragmentainers).

The effect on intrinsic sizing is under discussion. <https://github.com/w3c/csswg-drafts/issues/2945>

The effect on multi-column containers is to discard any overflow column.

Note: In the case of parallel fragmentation flows, content occuring 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.

Given an article with one excessively long overflowing line, and four more lines than can fit in the

as in the illustration bellow, different renderings are possible based on the combination of the

and

property.

This specification introduces no new privacy or security concerns.

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.

Conformance requirements are expressed with a combination of descriptive assertions and RFC 2119 terminology. The key words “MUST”, “MUST NOT”, “REQUIRED”, “SHALL”, “SHALL NOT”, “SHOULD”, “SHOULD NOT”, “RECOMMENDED”, “MAY”, and “OPTIONAL” in the normative parts of this document are to be interpreted as described in RFC 2119. However, for readability, these words do not appear in all uppercase letters in this specification.

All of the text of this specification is normative except sections explicitly marked as non-normative, examples, and notes. [RFC2119]

Examples in this specification are introduced with the words “for example” or are set apart from the normative text with class="example", like this:

Informative notes begin with the word “Note” and are set apart from the normative text with class="note", like this:

Note, this is an informative note.

Advisements are normative sections styled to evoke special attention and are set apart from other normative text with <strong class="advisement">, like this: UAs MUST provide an accessible alternative.

A style sheet is conformant to this specification if all of its statements that use syntax defined in this module are valid according to the generic CSS grammar and the individual grammars of each feature defined in this module.

A renderer is conformant to this specification if, in addition to interpreting the style sheet as defined by the appropriate specifications, it supports all the features defined by this specification by parsing them correctly and rendering the document accordingly. However, the inability of a UA to correctly render a document due to limitations of the device does not make the UA non-conformant. (For example, a UA is not required to render color on a monochrome monitor.)

An authoring tool is conformant to this specification if it writes style sheets that are syntactically correct according to the generic CSS grammar and the individual grammars of each feature in this module, and meet all other conformance requirements of style sheets as described in this module.

The following sections define several conformance requirements for implementing CSS responsibly, in a way that promotes interoperability in the present and future.

So that authors can exploit the forward-compatible parsing rules to assign fallback values, CSS renderers must treat as invalid (and ignore as appropriate) any at-rules, properties, property values, keywords, and other syntactic constructs for which they have no usable level of support. In particular, user agents must not selectively ignore unsupported property values and honor supported values in a single multi-value property declaration: if any value is considered invalid (as unsupported values must be), CSS requires that the entire declaration be ignored.

Once a specification reaches the Candidate Recommendation stage, implementers should release an unprefixed implementation of any CR-level feature they can demonstrate to be correctly implemented according to spec, and should avoid exposing a prefixed variant of that feature.

To establish and maintain the interoperability of CSS across implementations, the CSS Working Group requests that non-experimental CSS renderers submit an implementation report (and, if necessary, the testcases used for that implementation report) to the W3C before releasing an unprefixed implementation of any CSS features. Testcases submitted to W3C are subject to review and correction by the CSS Working Group.

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