CSS Exclusions and Shapes Module Level 3

Abstract

CSS 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 document

This is a public copy of the editors' draft. It is provided for discussion only and may change at any moment. Its publication here does not imply endorsement of its contents by W3C. Don't cite this document other than as 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.

Table of contents

• 1. Introduction
• 2. Definitions
• 3. Exclusions
  • 3.1. Declaring exclusions
    • 3.1.1. The ‘wrap-flow’ property
  • 3.2. Scope and effect of exclusions
  • 3.3. Propagation of Exclusions
    • 3.3.1. The ‘wrap-through’ Property
  • 3.4. Exclusions order
  • 3.5. Processing model
    • 3.5.1. Description
    • 3.5.2. Step 1: resolve exclusion boxes belonging to each wrapping context
    • 3.5.3. Step 2: resolve wrapping contexts and lay out containing blocks
    • 3.5.4. Step 2-A: resolve the position and size of exclusion boxes
    • 3.5.5. Step 2-B: lay out containing block applying wrapping
    • 3.5.6. Example
      • 3.5.6.1. Step 1: resolve exclusion boxes belonging to each wrapping context
      • 3.5.6.2. Step 2: resolve wrapping contexts and lay out containing blocks
        • 3.5.6.2.1. Resolving RWC-1
        • 3.5.6.2.2. Resolving RWC-2
  • 3.6. Floats and exclusions
    • 3.6.1. Similarities
    • 3.6.2. Differences
    • 3.6.3. Interoperability
      • 3.6.3.1. Effect of floats on exclusions
      • 3.6.3.2. Effect of exclusions on floats
• 4. Shapes
  • 4.1. Relation to the box model and float behavior
  • 4.2. Basic Shapes
    • 4.2.1. Supported Shapes
    • 4.2.2. Referencing SVG shapes
    • 4.2.3. Interpolation of Basic Shapes
  • 4.3. Shapes from Image
  • 4.4. Declaring Shapes
    • 4.4.1. The ‘shape-outside’ Property
    • 4.4.2. The ‘shape-inside’ Property
    • 4.4.3. The ‘shape-image-threshold’ Property
    • 4.4.4. The ‘shape-margin’ property
    • 4.4.5. The ‘shape-padding’ Property
• 5. Conformance
• Acknowledgments
• References
  • Normative references
  • Other references
• Index
• Property index
• Change Log
  • Since May 3rd 2012
  • Since December 13th 2011

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 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

The area used for excluding inline flow content around an exclusion box. The exclusion area is equivalent to the border box for an exclusion box. This specification's ‘shape-outside’ property can be used to define arbitrary, non-rectangular exclusion areas. The ‘shape-inside’ property also defines an exclusion area, but in this case it is the area outside the shape that inline content avoids.

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

The wrapping context of a box is a collection of exclusion areas contributed by its associated exclusion boxes and elements with ‘shape-inside’. During layout, a box wraps its inline flow content in the wrapping 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 content area is normally used for layout of the inline flow content of a box.

Wrapping area

The area used for layout of inline flow content of a box affected by a wrapping context, defined by subtracting the wrapping context from its content area

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 wrap around the exclusion 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 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

No exclusion is created. Inline flow content interacts with the element as usual. In particular, if the element is a float (see [CSS21]), the behavior is unchanged.

both

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

start

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

end

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

minimum

Inline flow content can flow around the edge of the exclusion with the smallest available space within the flow content's containing block, and must leave the other edge of the exclusion empty.

maximum

Inline flow content can flow around the edge of the exclusion with the largest available space within the flow content's containing block, and must leave the other edge of the exclusion empty.

clear

Inline flow content can only flow before and after the exclusion in the flow content's block direction and must leave the areas next to the start and end edges of the exclusion empty.

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’, ‘minimum’, ‘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.

Exclusion with ‘wrap-flow: start’ interacting with various writing modes.

Determining the relevant edges of the exclusion depends on the writing mode [CSS3-WRITING-MODES] of the content wrapping around the 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’ computed 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 of left-to-right writing mode content because the ‘wrap-flow’ for ‘B’ is ‘start’.

The background ‘blue’ area shows what areas are available for a left-to-right writing mode element's inline content layout. All areas represented with a light or dark shade of gray are not available for (left-to-right writing mode) inline content layout.

The ‘wrap-flow’ property values applied to exclusions as grid items.

    
<div id="grid">
    <div id="top-right" class="exclusion"></div>
    <div id="bottom-left" class="exclusion"></div>
    <div id="content">Lorem ipsum…</div>
</div>

<style type="text/css"> 
#grid {
    width: 30em;
    height: 30em;
    display: grid;
    grid-columns: 25% 25% 25% 25%;
    grid-rows: 25% 25% 25% 25%;

#top-right {
    grid-column: 3;
    grid-row: 2;
}

#bottom-left {
    grid-column: 2;
    grid-row: 3;
}

.exclusion {
    wrap-flow: <see below>
}

#content {
    grid-row: 1;
    grid-row-span: 4;
    grid-column: 1;
    grid-column-span: 4;
}
</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).

.exclusion{ wrap-flow: auto; }	.exclusion{ wrap-flow: both; }
.exclusion{ wrap-flow: start; }	.exclusion{ wrap-flow: end; }
.exclusion{ wrap-flow: minimum; }	.exclusion{ wrap-flow: maximum; }
.exclusion{ wrap-flow: clear; }

3.2. Scope and effect of exclusions

An exclusion affects the inline flow content descended from the exclusion's containing block (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).

As a reminder, for exclusions with ‘position:fixed’, the containing block is that of the root element.

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.

3.3.1. The ‘wrap-through’ Property

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"> 
    #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>

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. 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.

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>

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

3.5. Processing model

3.5.1. Description

Applying exclusions is a two-step process:

• Step 1: resolve exclusion boxes belonging to each wrapping context
• Step 2: resolve wrapping contexts and lay out each containing block in turn:
  • Step 2-A: resolve the position and size of exclusion boxes
  • Step 2-B: lay out containing block, wrapping around exclusion areas

3.5.2. Step 1: resolve exclusion boxes belonging to each wrapping context

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.

3.5.3. Step 2: resolve wrapping contexts and lay out containing blocks

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 boxes

Resolving 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. Each exclusion box is processed in turn, starting from the top-most, and each exclusion area is computed and contributed to the containing block's wrapping context.

Scrolling is ignored in this step when resolving the position and size of ‘position:fixed’ exclusion boxes.

Once the containing block's wrapping context is computed, all exclusion boxes in that wrapping context are removed from the normal flow.

3.5.5. Step 2-B: lay out containing block applying wrapping

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. Example

This 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 {
    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 {
    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:

• the document's DOM tree
• the layout tree of generated block boxes

DOM tree

Layout tree of generated block boxes

3.5.6.1. Step 1: resolve exclusion boxes belonging to each wrapping context

The 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:

• The wrapping context for the html element contains the e2 box: WC-1 (Wrapping Context 1)
• The wrapping context for the body element inherits the html element's wrapping context: WC-1
• The wrapping context for 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-2
• The wrapping context for d2 inherits the body element's wrapping context: WC-1

3.5.6.2. Step 2: resolve wrapping contexts and lay out containing blocks

In this step, each containing block is processed in turn. For each containing block, we (conceptually) go through two phases:

1. resolve the wrapping context: resolve the position and size of its exclusions
2. lay out the containing block

In our example, this breaks down to:

1. resolve the position and size of the exclusions belonging to WC-1: RWC-1 (Resolved Wrapping Context 1).
2. lay out the initial containing block (i.e., lay out its content):
   1. resolve the html element's wrapping context: RWC-1
   2. lay out the html element:
      1. resolve the body element's wrapping context: RWC-1
      2. lay out the body element:
         1. resolve the d1 element's wrapping context: RWC-2
         2. lay out the d1 element
         3. resolve the d2 element's wrapping context: RWC-1
         4. lay out the d2 element

3.5.6.2.1. Resolving RWC-1

The 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.

3.5.6.2.2. Resolving RWC-2

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 d1’s box, a first layout of d1 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. Similarities

There are similarities between floats and exclusions in that inline content wraps around floats and also wraps around exclusion areas. However, there are very significant differences.

3.6.2. Differences

• scope. While floats apply to content that follows in the document, exclusions apply to content in their containing block.
• positioning. Floats are part of the inline flow and ‘float’ 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.
• separation of concerns. Making an element a float determines both its positioning scheme and its effect on inline content. Making an element an exclusion only determines its impact on inline content and does not impose constraints on its positioning method.

3.6.3. Interoperability

3.6.3.1. Effect of floats on exclusions

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 floats

Exclusions 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

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 behavior

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 [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>

4.2. Basic Shapes

Shapes can be specified using syntax similar to SVG's basic shapes. The definitions use <length> type and the <percentage> types (see [CSS3VAL]). Percentages are resolved from the computed value of the ‘box-sizing’ property [CSS3UI] on 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]). Path styling like stroking is not considered part of the specified shape.

4.2.1. Supported Shapes

The following basic shapes are supported.

rectangle([<length>|<percentage>]{4} [ curve [<length>|<percentage>]{1,2} ]? )

• The four values represent x, y, width and height. They define the bounding box of the rectangle. Negative values for width and height are invalid.
• The two ‘curve’ values represent rx and ry. For rounded rectangles they define the x-axis radius and y-axis radius of the ellipse used to round off the corners of the rectangle. Negative values for rx and ry are invalid.

circle([<length>|<percentage>]{3})

• The three values values represent cx, cy, and r. They define the x-axis and y-axis coordinates of the center of the circle and the radius of the circle. A negative value for r is invalid.

ellipse([<length>|<percentage>]{4})

• The four values represent cx, cy, rx, and ry. They define the x-axis and y-axis coordinates of the center of the ellipse and the x-axis and y-axis radius of the ellipse. Negative values for rx and ry are invalid.

polygon([<fill-rule>,]? [<length>|<percentage>]{2}#)

• fill-rule - 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.
• The two length values represent xi and yi - the x and y axis coordinates of the i-th vertex of the polygon.

The UA will close a polygon by connecting the last vertex with the first vertex of the list.

4.2.2. 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.

<style>
div {
    height: 400px;
    width: 400px;
}
.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="M 100 100 L 300 100 L 200 300 z" />
</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, the relevant box is determined by the computed value of the ‘box-sizing’ property. All the lengths expressed in percentages are resolved from the relevant box. The coordinate system for the shape has its origin on the top-left corner of the relevant 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 relevant 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.

4.2.3. Interpolation of Basic Shapes

For interpolating between one basic shape and a second, the rules described below are applied.

• If both basic shapes are of the same type and not of type polygon:
  • Interpolate between each value.
• If one basic shape is of type rectangle and the second of type circle or ellipse:
  • Replace the circle/ellipse with a temporary rectangle of equal dimension.
  • Set the values for rx and ry on the temporary rectangle to ‘50%’.
  • Interpolate between the two rectangles as above.
• If both basic shapes are of type polygon and if both polygons have the same number of vertices:
  • Interpolate between each value.
• In all other cases:
  • No interpolation is specified.

4.3. 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.

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.

An image is floating to the left of a paragraph. The image shows the 3D version of the CSS logo over a transparent background. The logo has a shadow using an alpha-channel.

The image defines its float area through the ‘shape-outside’ property and specifies a value of 35 pixels for the ‘shape-margin’ property.

<p>
  <img id="CSSlogo" src="CSS-logo1s.png"/>
  blah blah blah blah...
</p>

<style>
  #CSSlogo {
    float: left;
    shape-outside: url("CSS-logo1s.png");
    shape-image-threshold: 0.1;
    shape-margin: 35px;
  }
</style>

The image needs two references to the image because this example uses the same image

1. to render it
2. as a shape defining the float area of the image

It is perfectly possible to display an image and use a different image for its float area.

In the figure below, the alpha-channel threshold is represented by the dotted line around the CSS logo and the 35px shape-margin is visible between that line and the edges of each individual line of the paragraph.

It's then possible to affect where the lines of the paragraph start in three ways:

1. Changing the value of the ‘shape-margin’ property
2. Changing the value of the ‘shape-image-threshold’ property
3. Modifying the alpha channel in the image

A float shape around an image using its alpha-channel with a 35 pixels shape-margin

4.4. Declaring Shapes

Shapes are declared with the ‘shape-outside’ or ‘shape-inside’ properties, with possible modifications from the ‘shape-margin’ and ‘shape-padding’ properties. The shape defined by the ‘shape-outside’ and ‘shape-margin’ properties changes the geometry of an exclusion element‘s exclusion area 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 defined by the ‘shape-inside’ and ‘shape-padding’ properties defines an exclusion area that contributes to the element's wrapping context. The ‘shape-inside’ property applies to all block-level elements.

4.4.1. The ‘shape-outside’ Property

The values of this property have the following meanings:

auto

The shape is computed based on the border box of the element for exclusions, or computed based on the margin box for floats.

<basic-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

4.4.2. The ‘shape-inside’ Property

The ‘shape-inside’ property adds one or more exclusion areas to the element's wrapping context. This modifies the normal rectangular shape of the content area to a possibly non-rectangular wrapping area. The exclusion areas are defined by subtracting the shape from the element's content area. Any part of the shape outside the element's content area has no effect.

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. E.g., when shape-outside computes to ‘auto’, use the meaning of ‘auto’ below to compute the shape.

auto

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

<basic-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-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’.

Effect of shape-inside on inline content.

Overflow content avoids the exclusion area(s) added by ‘shape-inside’ and ‘shape-padding’ (as well as any other exclusion areas in the element's wrapping context).

Overflow interacting with exclusion areas defined by ‘shape-inside’ and ‘shape-padding’.

4.4.3. The ‘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:

<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.

4.4.4. The ‘shape-margin’ property

The ‘shape-margin’ property adds a margin to a shape-outside. This defines a new shape where every point is the specified distance from the shape-outside. This property takes on positive values only.

A ‘shape-margin’ creating an offset from a circlular shape-outside. The blue rectangles represent inline content affected by the shape created by the margin.

4.4.5. The ‘shape-padding’ Property

The ‘shape-padding’ property adds padding to a shape-inside. This defines a new shape where every point is the specified distance from the shape-inside. This property takes on positive values only.

A ‘shape-padding’ creating an offset from a circlular shape-inside. The light blue rectangles represent inline content affected by the shape created by the padding.

5. Conformance

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.

Conformance classes

Conformance to this specification 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 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.

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.

Experimental implementations

To avoid clashes with future CSS features, the CSS 2.1 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.

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.

Acknowledgments

This specification is made possible by input from Andrei Bucur, Alexandru Chiculita, Arron Eicholz, Daniel Glazman, Arno Gourdol, Chris Jones, Bem Jones-Bey, Marcus Mielke, Alex Mogilevsky, Hans Muller, Mihnea Ovidenie, Virgil Palanciuc, Peter Sorotokin, Bear Travis, Eugene Veselov, Stephen Zilles 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

[CSS3-WRITING-MODES]

Elika J. Etemad; Koji Ishii. CSS Writing Modes Module Level 3. 15 November 2012. W3C Working Draft. (Work in progress.) URL: http://www.w3.org/TR/2012/WD-css3-writing-modes-20121115/

[CSS3BOX]

Bert Bos. CSS basic box model. 9 August 2007. W3C Working Draft. (Work in progress.) URL: http://www.w3.org/TR/2007/WD-css3-box-20070809

[CSS3UI]

Tantek Çelik. CSS Basic User Interface Module Level 3 (CSS3 UI). 17 January 2012. W3C Working Draft. (Work in progress.) URL: http://www.w3.org/TR/2012/WD-css3-ui-20120117/

[CSS3VAL]

Håkon Wium Lie; Tab Atkins; Elika J. Etemad. CSS Values and Units Module Level 3. 28 August 2012. W3C Candidate Recommendation. (Work in progress.) URL: http://www.w3.org/TR/2012/CR-css3-values-20120828/

[RFC2119]

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

[SVG11]

Erik Dahlström; et al. Scalable Vector Graphics (SVG) 1.1 (Second Edition). 16 August 2011. W3C Recommendation. URL: http://www.w3.org/TR/2011/REC-SVG11-20110816/

Other references

[CSS3-FLEXBOX]

Tab Atkins Jr.; Elika J. Etemad; Alex Mogilevsky. CSS Flexible Box Layout Module. 18 September 2012. W3C Candidate Recommendation. (Work in progress.) URL: http://www.w3.org/TR/2012/CR-css3-flexbox-20120918/

[CSS3-GRID-LAYOUT]

Alex Mogilevsky; et al. CSS Grid Layout. 6 November 2012. W3C Working Draft. (Work in progress.) URL: http://www.w3.org/TR/2012/WD-css3-grid-layout-20121106/

Index

• authoring tool, 5.
• circle, 4.2.1.
• Content area, 2.
• ellipse, 4.2.1.
• Exclusion area, 2.
• Exclusion box, 2.
• Exclusion element, 2.
• Float area, 2.
• outside-inside, 2.
• polygon, 4.2.1.
• rectangle, 4.2.1.
• renderer, 5.
• 'shape-image-threshold', 4.4.3.
  • '<alphavalue>', 4.4.3.
• 'shape-inside', 4.4.2.
  • 'auto', 4.4.2.
  • '<basic-shape>', 4.4.2.
  • 'outside-shape', 4.4.2.
  • '<uri>', 4.4.2.
• 'shape-margin', 4.4.4.
• 'shape-outside', 4.4.1.
  • 'auto', 4.4.1.
  • '<basic-shape>', 4.4.1.
  • '<uri>', 4.4.1.
• 'shape-padding', 4.4.5.
• style sheet
  • as conformance class, 5.
• 'wrap-flow', 3.1.1.
  • 'auto', 3.1.1.
  • 'both', 3.1.1.
  • 'clear', 3.1.1.
  • 'end', 3.1.1.
  • 'maximum', 3.1.1.
  • 'minimum', 3.1.1.
  • 'start', 3.1.1.
• Wrapping area, 2.
• Wrapping context, 2.
• 'wrap-through', 3.3.1.
  • 'none', 3.3.1.
  • 'wrap', 3.3.1.

Property index

CSS’, ‘Exclusions’);

Change Log

Since May 3rd 2012

• Changed shape-inside overflow diagrams to show exclusion behavior
• Changed shape-inside to contribute to the wrapping context
• Defined exclusion edges relative to wrapping content's writing mode
• Made use of start, end, before and after consistent
• Added interpolation for basic shapes
• Changed basic shapes to depend on box specified with box-sizing
• Added overflow behavior for shape-inside.
• Added wrap-flow:minimum.
• Clarified processing model.
• Changed wrap-margin and wrap-padding to shape-margin and shape-padding.
• Removed wrap shorthand.

Since December 13th 2011

• Clarified processing model.
• Clarified interaction with floats.
• Clarified that an exclusion element establishes a new block formatting context.