CSS Exclusions and Shapes Module Level 3

Abstract

CSS exclusions define arbitrary areas around which inline content can flow. Unlike CSS floats, which they extend, CSS exclusions can be positioned with any CSS positioning schemes.

CSS Shapes control the geometric shapes used for wrapping inline flow content outside or inside an element.

Combining CSS Exclusions and CSS Shapes allows sophisticated layouts, for example having content flow into and/or around circles or other, arbitrarily complex shapes.

Status of this document

This 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.

The (archived) public mailing list www-style@w3.org (see instructions) is preferred for discussion of this specification. When sending e-mail, please put the text “css3-exclusions” in the subject, preferably like this: “[css3-exclusions] …summary of comment…”

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.

This is the First Public Working Draft of the CSS Exclusions and Shapes Level 3 Module.

Table of contents

1. Introduction

This 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. Definitions

Exclusion element

An element that defines an exclusion area for other elements. The ‘wrap-flow’ property is used to make an element an exclusion element. An exclusion element contributes its exclusion area to its containing block's wrapping context

Exclusion area

The area used for excluding inline flow content around an exclusion element. The exclusion area is equivalent to the border box for elements with ‘float’ property computed to ‘none’ and the margin box for elements with ‘float’ property computed to a value other than ‘none’. This specification's ‘shape-outside’ property can be used to define arbitrary, non-rectangular exclusion areas.

Wrapping context

The wrapping context of an element is a collection of ‘exclusion areas’. The wrapping context is used to wrap around inline flow content during layout. An element will wrap its inline flow content in the area that corresponds to the subtraction of its wrapping context from its own content area.

An element 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 an element. 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.

3. Exclusions

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 avoid 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 is used to change the shape of exclusions.

3.1. Declaring exclusions

An element becomes an exclusion when its ‘wrap-flow’ property has a computed value other than ‘auto’.

3.1.1. The ‘wrap-flow’ property

The values of this property have the following meanings:

auto

For floats an exclusion is created, for all other elements an exclusion is not created.

both

Inline flow content can flow on all sides of the exclusion.

start

Inline flow content can wrap on the start edge of the exclusion area but must leave the area to end edge of the exclusion area empty.

end

Inline flow content can wrap on the end side of the exclusion area but must leave the area to the start edge of the exclusion area empty.

maximum

Inline flow content can wrap on the side of the exclusion with the largest available space for the given line, and must leave the other side of the exclusion empty.

clear

Inline flow content can only wrap on top and bottom of the exclusion and must leave the areas to the start and end edges of the exclusion box empty.

Setting the ‘wrap-flow’ property to ‘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.

The initial value for this property is ‘auto’.

When the property's computed value is ‘auto’, the element does not become an exclusion element unless its ‘float’ property computed value is not ‘none’. In that case, the element contributes its ‘border box’ to its containing block's wrapping context and content flows around it according to the ‘clear’ property.

The element will be considered as an exclusion for all inline flow content descendants of the exclusions' containing block.

Combining exclusions

3.2. Scope and Effect of Exclusions

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 any element, the ‘wrap-through’ property can be used (see definition of Propagation of Exclusions below).

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.

The ‘wrap-padding’ property can be used to offset to 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.

3.3. Propagation of Exclusions

By 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.

The values of this property have the following meanings:

wrap

The element inherits its parent node's wrapping context. Its descendant inline content wraps around exclusions defined outside the element.

none

The element does not inherit its parent node's wrapping context. Its descendants are only subject to exclusion shapes defined inside the element.

Using the ‘wrap-through’ property to control the effect of exclusions.

<style type="text/css"> 
    .exclusion  {
    	wrap-flow: both; 
    	position: absolute; 
    	background-color: rgba(220, 230, 242, 0.5); 
    } 
</style> 

<div style=”position: relative;”> 
    <div class=”exclusion”></div> 
    <div style=”wrap-through: wrap;”> Lorem ipsum dolor sit amet...</div> 
    <div style=”wrap-through: none;”> Lorem ipsum dolor sit amet...</div> 
</div>

3.3.2. The ‘wrap’ Shorthand Property

The ‘wrap’ property is a shorthand property for setting the exclusions properties at the same place in the style sheet.

3.4. Exclusions order

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. To change the ordering of exclusions with ‘position’ property computed to a value other than ‘static’, ‘z-index’ can be used. Exclusions with ‘position’ property computed to ‘static’ are not affected by the ‘z-index’ property, thus follow the painting order.

Ordering of exclusions.

<style type="text/css"> 
    .exclusion  {
    	wrap-flow: both; 
    	position: absolute; 
    	width: 50%; 
    	height: 50%; 
    } 
</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>

#orderedExclusion{ z-index: auto; }	#orderedExclusion{ z-index: 1; }

3.5. Exclusions implementation note

This section is not normative.

Exclusions can be specified on positioned elements and elements can be positioned from their static position. Since, the static position of such elements depends on the inline flow content affected by the exclusion itself; there is a circular dependency between the two layout tasks. To break this circular dependency a two-pass layout approach is recommended. The first layout pass computes the static positions of all auto-positioned exclusions by laying out all content besides exclusions. The second pass is layout of all elements including all exclusions. Exclusions that depend on their static position will use the position calculated during the first layout pass.

Note, this may cause exclusions to overlap or be distant from their static position (compared to non-exclusions auto-positioned elements).

Similar dependency exists for exclusions whose size or position is specified in percentage and the containing block size is specified as ‘auto’. In such cases the size or position of the exclusion is calculated based on the size of the containing block calculated in the first layout pass.

Exclusions can be positioned on all levels of nested elements. If layout is restarted on every level of nesting, the time to complete layout will be exponential. To avoid multiple layout passes, the restart of the second layout pass should be scoped to the top-most containing block of exclusions. Similarly, restarting layout for the entire document is not necessary unless there are exclusions whose containing block is the initial containing block.

4. Shapes

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. The inside shape defines an element's content shape and the element's inline content will flow within that shape.

Flow Container

A flow container is an element with a ‘display’ value that is computed as ‘block’, ‘table-cell’ or ‘inline-block’.

Note, while 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 [[CSS3-Box]] module.

CSS ‘shape’ and CSS box model relation.

<style type="text/css"> 
    .exclusion  {
    	wrap-flow: both; 
    	position: absolute; 
    	shape-outside: circle(50%, 50%, 50%); 
    	border: 1px solid red; 
    } 
</style> 

<div style=”position: relative; border: 1px solid black;”> 
    <div class=”exclusion”></div> 
    Lorem ipsum dolor sit amet... 
</div>

4.1. Shapes from SVG Syntax

Shapes can be specified using SVG basic shapes.

4.1.1. Supported SVG Shapes

The following SVG shapes are supported by the CSS shapes module.

rectangle(x, y, width, height, [[rx], ry])

• x, y, width and height - define the bounding box of the rectangle
• rx - For rounded rectangles, the x-axis radius of the ellipse used to round off the corners of the rectangle
• ry - For rounded rectangles, the y-axis radius of the ellipse used to round off the corners of the rectangle

circle(cx, cy, r)

• cx - The x-axis coordinate of the center of the circle
• cy - The y-axis coordinate of the center of the circle
• r - The radius of the circle

ellipse(cx, cy, rx, ry)

• cx - The x-axis coordinate of the center of the ellipse
• cy - The y-axis coordinate of the center of the ellipse
• rx - The x-axis radius of the ellipse
• ry - The y-axis radius of the ellipse

polygon([fillRule], x1, y1, x2, y2, x3, y3, … xn, yn)

• fillRule - The filling rule used to determine the interior of the polygon. See fill-rule property in SVG for details. Possible values are nonzero or evenodd. Default value when omitted is nonzero.
• xi - The x-axis coordinate of the i-th vertex of the polygon
• yi - The y-axis coordinate of the i-th vertex of the polygon

If the polygon is not closed the user-agent will automatically add a new vertex at the end.

4.1.2. Referencing SVG shapes

An SVG shape can be referenced using the url() syntax. The shape can be any of the SVG basic shapes or a path element.

In all cases, percentages are resolved from the border box of the 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%x" 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 corder 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 are computed agains ‘0’ and resolve to 0.

4.2. Shapes from Image

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.

4.3. Declaring Shapes

Shapes are declared with the ‘shape-outside’ or ‘shape-inside’ properties. The ‘shape-outside’ property changes the geometry of an exclusion element‘s exclusion are. If the element is not an exclusion element (see the ’wrap-flow' property), 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 values of this property have the following meanings:

auto

The shape is computed based on the border box of the element.

<shape>

The shape is computed based on the values of one of ‘rectangle’,‘ circle’, ‘ellipse’ or ‘polygon’.

<uri>

If the <uri> references an SVG shape element, that element defines the shape. Otherwise, if the <uri> references an image, the shape is extracted and computed based on the alpha channel of the specified image. If the <uri> does not reference an SVG shape element or an image, the effect is as if the value ‘auto’ had been specified.

Arbitrary shapes for exclusions

The ‘shape-inside’ modifies the shape of the inner inline flow content from rectangular content box to an arbitrary geometry.

The values of this property have the following meanings:

outside-shape

The shape is computed based on the computed value of the ‘shape-outside’ property.

auto

The shape is computed based on the content box of the element.

<shape>

The shape is computed based on the values of one of ‘rectangle’,‘ circle’, ‘ellipse’ or ‘polygon’.

<uri>

If the <uri> references an SVG shape element, that element defines the shape. Otherwise, if the <uri> references an image, the shape is extracted and computed based on the alpha channel of the specified image. If the <uri> does not reference an SVG shape element or an image, the effect is as if the value ‘auto’ had been specified.

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:

<alphavalue>

A <number> value used to set the threshold used for extracting a shape from an image. Any values outside the range 0.0 (fully transparent) to 1.0 (fully opaque) will be clamped to this range.

5. Conformance

5.1. Document Conventions

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.

5.2. Conformance Classes

Conformance to CSS Exclusions and Shapes is defined for three conformance classes:

style sheet

A CSS style sheet.

renderer

A UA that interprets the semantics of a style sheet and renders documents that use them.

authoring tool

A UA that writes a style sheet.

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 Implementations

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 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 Implementations

To 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 Implementations

Once 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 Criteria

For 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:

independent

each implementation must be developed by a different party and cannot share, reuse, or derive from code used by another qualifying implementation. Sections of code that have no bearing on the implementation of this specification are exempt from this requirement.

interoperable

passing the respective test case(s) in the official CSS test suite, or, if the implementation is not a Web browser, an equivalent test. Every relevant test in the test suite should have an equivalent test created if such a user agent (UA) is to be used to claim interoperability. In addition if such a UA is to be used to claim interoperability, then there must one or more additional UAs which can also pass those equivalent tests in the same way for the purpose of interoperability. The equivalent tests must be made publicly available for the purposes of peer review.

implementation

a user agent which:

1. implements the specification.
2. is available to the general public. The implementation may be a shipping product or other publicly available version (i.e., beta version, preview release, or “nightly build”). Non-shipping product releases must have implemented the feature(s) for a period of at least one month in order to demonstrate stability.
3. is not experimental (i.e., a version specifically designed to pass the test suite and is not intended for normal usage going forward).

The specification will remain Candidate Recommendation for at least six months.

Acknowledgments

This specification is made possible by input from Stephen Zilles, Alexandru Chiculita, Andrei Bucur, Mihnea Ovidenie, Peter Sorotokin, Virgil Palanciuc, Alan Stearns, Arno Gourdol, Eugene Veselov, Arron Eicholz, Alex Mogilevsky, Chris Jones, Marcus Mielke, and the CSS Working Group members.

References

Normative references

[CSS21]

Bert Bos; et al. Cascading Style Sheets Level 2 Revision 1 (CSS 2.1) Specification. 7 June 2011. W3C Recommendation. URL: http://www.w3.org/TR/2011/REC-CSS2-20110607

[RFC2119]

S. Bradner. Key words for use in RFCs to Indicate Requirement Levels. Internet RFC 2119. URL: http://www.ietf.org/rfc/rfc2119.txt

Other references

Index

• authoring tool, 5.2.
• Content area, 2.
• Exclusion area, 2.
• Exclusion element, 2.
• Flow Container, 4.
• outside-inside, 2.
• renderer, 5.2.
• ‘shape-image-threshold’, 4.3.3.
• ‘shape-inside’, 4.3.2.
• ‘shape-outside’, 4.3.1.
• style sheet
  • as conformance class, 5.2.
• ‘wrap’, 3.3.2.
• ‘wrap-flow’, 3.1.1.
• ‘wrap-margin’, 3.2.1.
• ‘wrap-padding’, 3.2.2.
• Wrapping context, 2.
• ‘wrap-through’, 3.3.1.

Property index