[css3-exclusions] message topic"
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AbstractCSS Exclusions define arbitrary areas around which inline content ([CSS21]) content can flow. CSS Exclusions can be defined on any CSS block-level elements. CSS Exclusions extend the notion of content wrapping previously limited to floats.
CSS Shapes control the geometric shapes used for wrapping inline flow content outside or inside an element. CSS Shapes can be applied to any element. A circle shape on a float will cause inline content to wrap around the circle shape instead of the float's bounding box.
Combining CSS Exclusions and CSS Shapes allows sophisticated layouts, allowing interactions between shapes in complex positioning schemes.
Status of this documentThis section describes the status of this document at the time of its publication. Other documents may supersede this document. A list of current W3C publications and the latest revision of this technical report can be found in the W3C technical reports index at http://www.w3.org/TR/.
Publication as a Working Draft does not imply endorsement by the W3C Membership. This is a draft document and may be updated, replaced or obsoleted by other documents at any time. It is inappropriate to cite this document as other than work in progress.
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This document was produced by the CSS Working Group (part of the Style Activity).
This document was produced by a group operating under the 5 February 2004 W3C Patent Policy. W3C maintains a public list of any patent disclosures made in connection with the deliverables of the group; that page also includes instructions for disclosing a patent. An individual who has actual knowledge of a patent which the individual believes contains Essential Claim(s) must disclose the information in accordance with section 6 of the W3C Patent Policy.
Changes since the previous draft are listed in the section “Change Log.”
Table of contentsThis section is not normative.
The exclusions section of this specification defines features that allow inline flow content to wrap around outside the exclusion area of elements.
The shapes section of the specification defines properties to control the geometry of an element's exclusion area as well as the geometry used for wrapping an element's inline flow content.
2. DefinitionsExclusion box
A box ([CSS3BOX]) that defines an exclusion area for other boxes. The ‘wrap-flow’ property is used to make an element's generated box an exclusion box. An exclusion box contributes its exclusion area to its containing block's wrapping context. An element with a ‘float’ computed value other than ‘none’ does not become an exclusion.
Exclusion area
Bug-15087Interaction of floats and exclusions (Howcome's questions)
The area used for excluding inline flow content around an exclusion element. The exclusion area is equivalent to the border box. This specification's ‘shape-outside’ property can be used to define arbitrary, non-rectangular exclusion areas.
Float area
The area used for excluding inline flow content around a float element. By default, the float area is the float element's margin box. This specification's ‘shape-outside’ property can be used to define arbitrary, non-rectangular float areas.
Exclusion element
An block-level element which is not a float and generates an exclusion box. An element generates an exclusion box when its ‘wrap-flow’ property's computed value is not ‘auto’.
Wrapping context
Bug-15086should the wrapping context be generic and include floats?
The wrapping context of a box is a collection of exclusion areas contributed by its associated exclusion boxes. During layout, a box wraps its inline flow content in the area that corresponds to the subtraction of its wrapping context from its own content area.
A box inherits its containing block's wrapping context unless it specifically resets it using the ‘wrap-through’ property.
Content area
The area used for layout of the inline flow content of a box. By default the area is equivalent to the content box. This specification's ‘shape-inside’ property can define arbitrary, non-rectangular content areas.
Outside and inside
In this specification, ‘outside’ refers to DOM content that is not a descendant of an element while ‘inside’ refers to the element's descendants.
Exclusion elements define exclusion areas that contribute to their containing block's wrapping context. As a consequence, exclusions impact the layout of their containing block's descendants.
Elements layout their inline content in their content area and wrap around the areas in their associated wrapping context. If the element is itself an exclusion, it does not wrap around its own exclusion shape and the impact of other exclusions on other exclusions is controlled by the ‘z-index’ property as explained in the exclusions order section.
The shape properties can be used to change the shape of exclusion areas.
3.1. Declaring exclusionsAn element becomes an exclusion when its ‘wrap-flow’ property has a computed value other than ‘auto’.
wrap-flow’ property Name: wrap-flow Value: auto | both | start | end | maximum | clear Initial: auto Applies to: block-level elements. Inherited: no Percentages: N/A Media: visual Computed value: as specified except for element's whose ‘float’ computed value is not none, in which case the computed value is ‘auto’. Bug-16437
Consistent use of start/end/top/bottom
The values of this property have the following meanings:
If the property's computed value is ‘auto’, the element does not become an exclusion.
Otherwise, a computed ‘wrap-flow’ property value of ‘both’, ‘start’, ‘end’, ‘maximum’ or ‘clear’ on an element makes that element an exclusion element. It's exclusion shape is contributed to its containing block's wrapping context, causing the containing block's descendants to wrap around its exclusion area.
An exclusion element establishes a new block formatting context (see [CSS21]) for its content.
Combining exclusions
The above figure illustrates how exclusions are combined. The outermost box represents an element's content box. The A, B, C and D darker gray boxes represent exclusions in the element's wrapping context. A, B, C and D have their respective ‘wrap-flow’ set to ‘both’, ‘start’, ‘end’ and ‘clear’ respectively. The lighter gray areas show the additional areas that are excluded for inline layout as a result of the ‘wrap-flow’ value. For example, the area to the right of ‘B’ cannot be used for inline layout because the ‘wrap-flow’ for ‘B’ is ‘start’.
The background ‘blue’ area shows what areas are available for the element's inline content layout. All areas represented with a light or dark shade of gray are not available for inline content layout.
Add an example with auto-height content and z-index for exclusions order.
Bug-15084Fluidity of the layout with respect to different amounts of content
The ‘wrap-flow’ property values applied to an absolutely positioned element.
<div id="grid">
<div id="exclusion">Donec metus …</div>
<div id="content">Lorem ipsum…</div>
</div>
<style type="text/css">
#grid {
width: 30em;
height: 30em;
display: grid;
grid-columns: 50% 25% 25%;
grid-rows: 40% 20% 40%;
}
#exclusion {
grid-row: 2;
grid-column: 2;
wrap-flow: <see below>
}
#content {
grid-row: 1;
grid-row-span: 3;
grid-column: 1;
grid-column-span: 3;
}
</style>
The following figures illustrate the visual rendering for different values of the ‘wrap-flow’ property. The gray grid lines are marking the grid cells. and the blue area is the exclusion box (positioned by the grid).
An exclusion affects the inline flow content descended from the exclusion's containing blocks (defined in CSS 2.1 10.1) and that of all descendant elements of the same containing block. All inline flow content inside the containing block of the exclusions is affected. To stop the effect of exclusions defined outside an element, the ‘wrap-through’ property can be used (see the propagation of exclusions section below).
For exclusions with ‘position:fixed’, the containing block is that of the root element.
wrap-margin’ property
The ‘wrap-margin’ property can be used to offset the inline flow content wrapping on the outside of exclusions. Offsets created by the ‘wrap-margin’ property are offset from the outside of the exclusion. This property takes on positive values only.
wrap-padding’ Property
The ‘wrap-padding’ property can be used to offset the inline flow content wrapping on the inside of elements. Offsets created by the ‘wrap-padding’ property are offset from the content area of the element. This property takes on positive values only.
Note that the ‘
wrap-padding
’ property affects layout of content inside the element it applies to while the ‘
wrap-margin
’ property affects layout of content outside the element.
3.3. Propagation of ExclusionsBy default, an element inherits its parent wrapping context. In other words it is subject to the exclusions defined outside the element.
Setting the ‘wrap-through’ property to ‘none’ prevents an element from inheriting its parent wrapping context. In other words, exclusions defined ‘outside’ the element, have not effect on the element's children layout.
Exclusions defined by an element's descendants still contribute to their containing block's
wrapping context. If that containing block is a child of an element with ‘
wrap-through
’ set to none, or the element itself, then exclusion still have an effect on the children of that containing block element.
3.3.1. The ‘wrap-through’ Property Name: wrap-through Value: wrap | none Initial: wrap Applies to: block-level elements Inherited: no Percentages: N/A Media: visual Computed value: as specified
The values of this property have the following meanings:
Using the ‘wrap-through’ property to control the effect of exclusions.
<style type="text/css">
#grid {
display: grid;
grid-columns: 25% 50% 25%;
grid-rows: 25% 25% 25% 25%;
}
#exclusion {
grid-row: 2;
grid-row-span: 2;
grid-column: 2;
wrap-flow: <see below>
}
#rowA, #rowB {
grid-row-span: 2;
grid-column: 1;
grid-column-span: 3;
}
#rowA {
grid-row: 1;
}
#rowB {
grid-row: 3;
}
</style>
<style type="text/css">
.exclusion {
wrap-flow: both;
position: absolute;
left: 20%;
top: 20%;
width: 50%;
height: 50%;
background-color: rgba(220, 230, 242, 0.5);
}
</style>
<div id="grid">
<div class=”exclusion”></div>
<div id="rowA" style=”wrap-through: wrap;”> Lorem ipsum dolor sit amet...</div>
<div id="rowB" style=”wrap-through: none;”> Lorem ipsum dolor sit amet...</div>
</div>
wrap’ Shorthand Property Name: wrap Value: <wrap-flow> || <wrap-margin> [ / <wrap-padding>] Initial: see individual properties Applies to: block-level elements Inherited: no Percentages: N/A Media: visual Computed value: see individual properties
The ‘wrap’ property is a shorthand property for setting the exclusions properties at the same place in the style sheet.
Exclusions follow the painting order (See [CSS21] Appendix E). Exclusions are applied in reverse to the document order in which they are defined. The last exclusion appears on top of all other exclusion, thus it affects the inline flow content of all other preceding exclusions or elements descendant of the same containing block. The ‘z-index’ property can be used to change the ordering of positioned exclusion boxes (see [CSS21]). Statically positioned exclusions are not affected by the ‘z-index’ property and thus follow the painting order.
Improve Example 3 about exclusion order
Ordering of exclusions.
<style type="text/css">
.exclusion {
wrap-flow: both;
position: absolute;
width: 50%;
height: auto;
}
</style>
<div class=”exclusion” style=”top: 0px; left: 0px;”>
Lorem ipsum dolor sit amet...
</div>
<div id="orderedExclusion" class=”exclusion” style=”top: 25%; left: 25%;”>
Lorem ipsum dolor sit amet...
</div>
<div class=”exclusion” style=”top: 50%; left: 50%;”>
Lorem ipsum dolor sit amet...
</div>
Is the CSS exclusions processing model incorrect?
Bug-15083Concerns over Error accumulation vs. performance
The rules for exclusions order and exclusions on absolutely positioned elements (particularly those with static position) build this exclusions model on top of the absolute positioning model in CSS Level 2, rather than on top of floats, the existing exclusion model in CSS Level 1 and 2. The CSS Working Group has not reached consensus on whether it intends to base the new exclusions features in this specification on top of absolute positioning (as these sections of this specification do) or on top of floats.
This module does not depend on any particular positioning scheme.
3.5. Processing model 3.5.1. DescriptionApplying exclusions is a two-step process:
In this step, the user agent determines which containing block each exclusion area belongs to. This is a simple step, based on the definition of containing blocks and elements with a computed value for ‘wrap-flow’ that is not auto.
In this step, starting from the top of the rendering tree (see [CSS21]), the the agent processes each containing block in two sub-steps.
3.5.4. Step 2-A: resolve the position and size of exclusion boxesResolving the position and size of exclusion boxes in the wrapping context may or may not require a layout. For example, if an exclusion box is absolutely positioned and sized, a layout may not be needed to resolve its position and size. In other situations, laying out the containing block's content is required.
When a layout is required, it is carried out without applying any exclusion area. In other words, the containing block is laid out without a wrapping context.
Step 2-A yields a position and size for all exclusion boxes in the wrapping context.
Scrolling is ignored in this step when resolving the position and size of ‘position:fixed’ exclusion boxes.
Finally, the content of the containing block is laid out, with the inline content wrapping around the wrapping content‘s exclusion areas (which may be different from the exclusion box because of the ’shape-outside' property).
When the containing block itself is an exclusion box, then rules on exclusions order define which exclusions affect the inline and descendant content of the box.
3.5.6. ExampleThis section illustrates the exclusions processing model with an example. It is meant to be simple. Yet, it contains enough complexity to address the issues of layout dependencies and re-layout.
The code snippet in the following example has two exclusions affecting the document's inline content.
<html>
<style>
#d1 {
position:relative;
height: auto;
color: #46A4E9;
border: 1px solid gray;
}
#e1 {
-webkit-wrap-flow: both;
position: absolute;
left: 50%;
top: 50%;
width: 40%;
height: 40%;
border: 1px solid red;
margin-left: -20%;
margin-top: -20%;
}
#d2 {
position: static;
width: 100%;
height: auto;
color: #808080;
}
#e2 {
-webkit-wrap-flow: both;
position: absolute;
right: 5ex;
top: 1em;
width: 12ex;
height: 10em;
border: 1px solid lime;
}
</style>
<body>
<div id="d1">
Lorem ipsusm ...
<p id="e1"></p>
</div>
<div id="d2">
Lorem ipsusm ...
<p id="e2" ></p>
</div>
</body>
</html>
The following figures illustrate:
DOM tree
Layout tree of generated block boxes
3.5.6.1. Step 1: resolve exclusion boxes belonging to each wrapping contextThe figures illustrate how the boxes corresponding to the element sometimes have a different containment hierarchy in the layout tree than in the DOM tree. For example, the box generated by e1 is positioned in its containing block's box, which is the d1-box, because e1 is absolutely positioned and d1 is relatively positioned. However, while e2 is also absolutely positioned, its containing block is the initial containing block (ICB). See the section 10.1 of the CSS 2.1 specification ([CSS21]) for details.
As a result of the computation of containing blocks for the tree, the boxes belonging to the wrapping contexts of all the elements can be determined:
e2 box: WC-1 (Wrapping Context 1)d1 inherits the body element's wrapping context and adds the e1-box to it. So the wrapping context is made of both the e1-box and the e2-box: WC-2d2 inherits the body element's wrapping context: WC-1In this step, each containing block is processed in turn. For each containing block, we (conceptually) go through two phases:
In our example, this breaks down to:
d1 element's wrapping context: RWC-2d1 elementd2 element's wrapping context: RWC-1d2 elementThe top-most wrapping context in the layout tree contains the e2 exclusion. Its position and size needs to be resolved. In general, computing an exclusion's position and size may or may not require laying out other content. In our example, no content needs to be laid out to resolve the e2 exclusion's position because it is absolutely positioned and its size can be resolved without layout either. At this point, RWC-1 is resolved and can be used when laying inline content out.
The process is similar: the position of the e1 exclusion needs to be resolved. Again, resolving the exclusion's position and size may require processing the containing block (d1 here). It is the case here because the size and position of e1 depend on resolving the percentage lengths. The percentages are relative to the size of d1‘s box. As a result, in order to resolve a size for ’s box, a first layout of d1d1 is done without any wrapping context (i.e., no exclusions applied). The layout yields a position and size for e1‘s box.
At this point, RWC-2 is resolved because the position and size of both e1 and e2 are resolved.
The important aspect of the above processing example is that once an element’s wrapping context is resolved (by resolving its exclusions' position and size), the position and size of the exclusions are not re-processed if the element's size changes between the layout that may be done without considering any wrapping context (as for RWC-2) and the layout done with the resolved wrapping context. This is what breaks the possible circular dependency between the resolution of wrapping contexts and the layout of containing blocks.
3.6. Floats and exclusions 3.6.1. SimilaritiesThere are similarities between floats an exclusions in that inline content wraps around floats and also wraps around exclusion areas. However, there are very significant differences.
3.6.2. Differencesfloat’ on the line box. Authors can control how the floats move on the line box, to the right or to the left. By contrast, exclusions can be positioned using any positioning scheme such as grid layout ([CSS3-GRID-LAYOUT]), flexible box ([CSS3-FLEXBOX]) or any other CSS positioning scheme.Floats have an effect on the positioning of exclusions and the layout of their inline content. For example, if an exclusion is an inline-box which happens to be on the same line as a float, its' position, as computed in Step 2-A will be impacted by the float, as is any other inline content.
3.6.3.2. Effect of exclusions on floatsExclusions have an effect on the positioning of floats as they have an effect on inline content. Therefore, in Step 2-B, floats will avoid exclusion areas.
4. Shapes Bug-16716Handling visible content as a shape for Exclusions
Shapes define arbitrary geometric contours around which or into which inline flow content flows. There are two different types of shapes – ‘outside’ and ‘inside’. The outside shape defines the exclusion area for an exclusion element or the float area for a float. The inside shape defines an element's content shape and the element's inline content will flow within that shape.
It is important to note that while outside shapes only apply to exclusions and floats, inside shapes apply to all block-level elements.
4.1. Relation to the box model and float behaviorWhile the boundaries used for wrapping inline flow content outside and inside an element can be defined using shapes, the actual box model does not change. If the element has specified margins, borders or paddings they will be computed and rendered according to the [CSS3BOX] module.
However, floats are an exception. If a float has an outside shape, its positioning is resolved as defined in [CSS21] but using the outside shape's bounding box is used in lieu of the float's margin box.
CSS ‘shape-outside’ and CSS box model relation: the red box illustrates the exclusion element's content box, which is unmodified and subject to normal CSS positioning (here absolute positioning).
<style type="text/css">
.exclusion {
wrap-flow: both;
position: absolute;
top: 25%;
left: 25%;
width: 50%;
height: 50%;
shape-outside: circle(50%, 50%, 50%);
border: 1px solid red;
}
</style>
<div style=”position: relative;”>
<div class=”exclusion”></div>
Lorem ipsum dolor sit amet...
</div>
In the following example the left and right floating div elements specify a triangular shape using the ‘shape-outside’ property.
<div style="text-align:center;">
<div id="float-left"></div>
<div id="float-right"></div>
<div>
Sometimes a web page's text content appears to be
funneling your attention towards a spot on the page
to drive you to follow a particular link. Sometimes
you don't notice.
</div>
</div>
<style type="text/css">
#float-left {
shape-outside: polygon(0,0 100%,100% 0,100%);
float: left;
width: 40%;
height: 12ex;
}
#float-right {
shape-outside: polygon(100%,0 100%,100% 0,100%);
float: right;
width: 40%;
height: 12ex;
}
</style>
</div>Shapes can be specified using SVG basic shapes. The definitions use <length> type and the <percentage> types (see [CSS3VAL]). Percentages are resolved from the border box of the element to which the property applies. For the radius r of the circle shape, a percentage value is resolved as specified in the SVG recommendation (see [SVG11])
The following SVG shapes are supported by the CSS shapes module.
curve’ values represent rx and ry - define the rounded rectangles, the x and y axis and radius of the ellipse used to round off the corners of the rectangleIf the polygon is not closed the user-agent will automatically add a new vertex at the end.
An SVG shape can be referenced using the url() syntax. The shape can be any of the SVG basic shapes or a path element.
<style>
.in-a-circle {
shape-inside: url(#circle_shape);
}
.in-a-path {
shape-inside: url(#path-shape);
}
</style>
<svg ...>
<circle id="circle_shape" cx="50%" cy="50%" r="50%" />
<path id="path-shape" d="..." />
</svg>
<div class="in-a-circle">...</div>
<div class="in-a-path">...</div>
When using the SVG syntax or referencing SVG elements to define shapes, all the lengths expressed in percentages are resolved from the border box of the element. The coordinate system for the shape has its origin on the top-left corner of the border box with the x-axis running to the right and the y-axis running downwards. If the SVG element uses unitless coordinate values, they are equivalent to using ‘px’ units. If the border box of the element is dependent on auto sizing (i.e., the element's ‘width’ or ‘height’ property is ‘auto’), then the percentage values resolve to 0.
Do we need to provide properties to repeat exclusion images as for the background-image property?
Bug-15090Use the contour() keyword. <img id=shape-me url=foo><style>#shape-me { shape-outside: contour; }</style> //equal to ‘shape-outside: url(foo)’ shape-outside: attr(src as url);
Specify what happens with animated images
Bug-16112Address security concern with automatic shape extractions for images
Another way of defining shapes is by specifying a source image whose alpha channel is used to compute the inside or outside shape. The shape is computed to be the path that encloses the area where the opacity of the specified image is greater than the ‘shape-image-threshold’ value. If the ‘shape-image-threshold’ is not specified, the initial value to be considered is 0.5.
Note, images can define cavities and inline flow content should wrap inside them. In order to avoid that, another exclusion element can be overlaid.
For animated raster image formats (such as GIF), the first frame of the animation sequence is used. For SVG images ([SVG11]), the image is rendered without animations applied.
4.4. Declaring ShapesShapes are declared with the ‘shape-outside’ or ‘shape-inside’ properties. The ‘shape-outside’ property changes the geometry of an exclusion element‘s exclusion area or or a float element’s float area. If the element is not an exclusion element (see the ‘wrap-flow’ property) or a float, then the ‘shape-outside’ property has no effect.
The ‘shape-inside’ property defines an element's content area and the element's inline flow content wraps into that shape. The ‘shape-inside’ property applies to all block-level elements.
shape-outside’ Property Name: shape-outside Value: auto | <shape> | <uri> Initial: auto Applies to: exclusion elements and floats Inherited: no Percentages: N/A Media: visual Computed value: computed lengths for <shape>, the absolute URI for <uri>, otherwise as specified
The values of this property have the following meanings:
rectangle’, ‘circle’, ‘ellipse’ or ‘polygon’.
auto’ had been specified.
Arbitrary shapes for exclusions
The above figure shows how ‘shape-outside’ shapes impact the exclusion areas. The red box represents an element's content box and ‘A’, ‘B’, ‘C’ and ‘C’ represent exclusions with a complex shape and their ‘wrap-flow’ property set to ‘both’, ‘start’, ‘end’ and ‘clear’, respectively.
As illustrated in the picture, when an exclusion allows wrapping on all sides, text can flow inside ‘holes’ in the exclusion (as for exclusion ‘A’). Otherwise, the exclusion clears the area on the side(s) defined by ‘wrap-flow’, as illustrated for ‘B’, ‘C’ and ‘D’ above.
shape-inside’ Property
The ‘shape-inside’ modifies the shape of the inner inline flow content from rectangular content box to an arbitrary geometry.
Should we revisit the decision to not allow SVG path syntax in the shape-inside, shape-outside properties
Bug-16460Specify overflow behavior for shape-inside
The values of this property have the following meanings:
shape-outside’ property.
rectangle’,‘ circle’, ‘ellipse’ or ‘polygon’.
auto’ had been specified.
The ‘shape-inside’ property applies to floats.
The ‘shape-inside’ property may not apply on some elements such as elements with a computed ‘display’ value of ‘table’.
shape-image-threshold’ Property
The ‘shape-image-threshold’ defines the alpha channel threshold used to extract the shape using an image. A value of 0.5 means that all the pixels that are more than 50% transparent define the path of the exclusion shape. The ‘shape-image-threshold’ applies to both ‘shape-outside’ and ‘shape-inside’.
The specified value of ‘shape-image-threshold’ is applied to both images used for ‘shape-outside’ and ‘shape-inside’.
The values of this property have the following meanings:
The shape properties apply to
5. Conformance 5.1. Document ConventionsConformance 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:
This is an example of an informative example.
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.
5.2. Conformance ClassesConformance to CSS Exclusions and Shapes is defined for three conformance classes:
A style sheet is conformant to CSS Exclusions and Shapes if all of its declarations that use properties defined in this module have values that are valid according to the generic CSS grammar and the individual grammars of each property as given in this module.
A renderer is conformant to CSS Exclusions and Shapes if, in addition to interpreting the style sheet as defined by the appropriate specifications, it supports all the features defined by CSS Exclusions and Shapes 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 CSS Exclusions and Shapes 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.
5.3. Partial ImplementationsSo 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 component 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.
5.4. Experimental ImplementationsTo avoid clashes with future CSS features, the CSS2.1 specification reserves a prefixed syntax for proprietary and experimental extensions to CSS.
Prior to a specification reaching the Candidate Recommendation stage in the W3C process, all implementations of a CSS feature are considered experimental. The CSS Working Group recommends that implementations use a vendor-prefixed syntax for such features, including those in W3C Working Drafts. This avoids incompatibilities with future changes in the draft.
5.5. Non-Experimental ImplementationsOnce a specification reaches the Candidate Recommendation stage, non-experimental implementations are possible, and implementors should release an unprefixed implementation of any CR-level feature they can demonstrate to be correctly implemented according to spec.
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.
Further information on submitting testcases and implementation reports can be found from on the CSS Working Group's website at http://www.w3.org/Style/CSS/Test/. Questions should be directed to the public-css-testsuite@w3.org mailing list.
5.6. CR Exit CriteriaFor this specification to be advanced to Proposed Recommendation, there must be at least two independent, interoperable implementations of each feature. Each feature may be implemented by a different set of products, there is no requirement that all features be implemented by a single product. For the purposes of this criterion, we define the following terms:
The specification will remain Candidate Recommendation for at least six months.
AcknowledgmentsThis specification is made possible by input from Andrei Bucur, Alexandru Chiculita, Arron Eicholz, Daniel Glazman, Arno Gourdol, Chris Jones, Marcus Mielke, Alex Mogilevsky, Mihnea Ovidenie, Virgil Palanciuc, Peter Sorotokin, Eugene Veselov, Stephen Zilles and the CSS Working Group members.
References Normative referencesshape-image-threshold’, 4.4.3.
<alphavalue>’, 4.4.3.shape-inside’, 4.4.2.
shape-outside’, 4.4.1.
wrap’, 3.3.2.wrap-flow’, 3.1.1.
wrap-margin’, 3.2.1.wrap-padding’, 3.2.2.wrap-through’, 3.3.1.
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