CSS Shapes Module Level 2

Editor's Draft 17 March 2014

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Alan Stearns, Adobe Systems, Inc.,
www-style@w3.org with subject line “[css-shapes] … message topic …” (archives)


This draft contains the features of CSS relating to wrapping content around and inside shapes. It (implicitly for now) includes and extends the functionality of CSS Shapes Level 1 [CSS-SHAPES]. The main points of extension compared to level 1 include additional ways of defining shapes, defining an exclusion area using a shape, and restricting an element's content area using a shape.

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 “www-style” in the subject, preferably like this: “[www-style] …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

This section is not normative.

Level 1 of this specification defined properties to control the geometry of an element's float area. This level defines how shapes apply to exclusions. It also includes a ‘shape-inside’ property for applying a shape to an element's content area. And finally it defines new ways of specifying shapes for all of these applications.

2. Terminology

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 wrapping 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.

Content area

The content area is normally used for layout of the inline flow content of a box.


shrink-to-fit circle / shape

3. Shapes

Shapes define arbitrary geometric contours around which inline content flows. The shape-outside property defines the float area for a float, and the exclusion area for an exclusion.

3.1. Basic Shapes

Add the final level 1 section with a possible addition of path() and a rejiggered rectangle().

rectangle() = rectangle( <>{4} [ round <<‘border-radius’>> ]? )

Should we revisit the decision to not allow SVG path syntax in the shape-inside, shape-outside properties

3.2. 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.

results of referencing SVG shapes
div {
    height: 400px;
    width: 400px;
.in-a-circle {
    shape-outside: url(#circle_shape);

.in-a-path {
    shape-outside: url(#path-shape);


<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" />

<div class="around-a-circle">...</div>
<div class="around-a-path">...</div>

3.3. Shapes from Image

Add the final level 1 section.

One suggestion is to define a shape based on an element's rendered content. This could have security implications.

Another suggestion is to add something to an image() function that determines the relevant pixels to use (both for defining a shape and for display).

3.4. Declaring Shapes

A shape can be declared with the ‘shape-outside’ property, with possible modifications from the ‘shape-margin’ property. The shape defined by the ‘shape-outside’ and ‘shape-margin’ properties changes the geometry of a float element's float area and an exclusion element's exclusion area.

A shape can be declared with the ‘shape-inside’ property, with possible modifications from the ‘shape-padding’ property. 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.

The red box illustrates an 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% at 50% 50%);
      border: 1px solid red;

<div style=”position: relative;”>
  <div class=”exclusion”></div>
  Lorem ipsum dolor sit amet...
Example rendering of circle shape and box model.

3.4.1. The ‘shape-outside’ Property

Add the final level 1 section with the change that shape-outside applies to block-level elements and has an effect if the element is an exclusion.

3.4.2. 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.

Name: shape-inside
Value: outside-shape | auto | <basic-shape> | <uri>
Initial: auto
Applies to: block-level elements
Inherited: no
Percentages: N/A
Media: visual
Computed value: computed lengths for <basic-shape>, the absolute URI for <uri>, otherwise as specified

The values of this property have the following meanings:

The shape is computed based on the content box of the element.
The shape is computed based on the values of one of ‘rectangle’, ‘inset-rectangle’, ‘circle’, ‘ellipse’ or ‘polygon’.
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’.

Content flowing with and without a shape-inside

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 rounded rect Overflow interacting with ellipse

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

When a shape-inside has a definite size (no percentages used in the shape's definition) an auto-sized element should use the shape as a constraint in determining its maximum size.

3.4.3. The ‘shape-image-threshold’ Property

Add the final level 1 section with the change that it applies to both shape-inside and shape-outside.

3.4.4. The ‘shape-image-source-type’ Property

Should we add an alpha/luminance switch to determine which values we use from the shape-image source? This could just be a keyword on the shape-image-threshold property. Whatever we go with should be compatible with the alpha/luminance switch from mask sources.

3.4.5. The ‘shape-margin’ property

Add the final level 1 section with the change that it applies to exclusions.

3.4.6. 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.

Name: shape-padding
Value: <length>
Initial: 0
Applies to: block-level elements
Inherited: no
Percentages: N/A
Media: visual
Computed value: the absolute length

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.

Example of a shape-padding offset

Note that the ‘shape-padding’ property only affects layout of content inside the element it applies to while the ‘shape-margin’ property only affects layout of content outside the element.

4. 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:

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.

Conformance classes

Conformance to this specification is defined for three conformance classes:

style sheet
A CSS style sheet.
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.



Normative references

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

Other references

Vincent Hardy; Rossen Atanassov; Alan Stearns. CSS Shapes Module Level 1. 11 February 2014. W3C Last Call Working Draft. (Work in progress.) URL: http://www.w3.org/TR/2014/WD-css-shapes-1-20140211/


Property index

Property Values Initial Applies to Inh. Percentages Media
shape-inside outside-shape | auto | <basic-shape> | <uri> auto block-level elements no N/A visual
shape-padding <length> 0 block-level elements no N/A visual

Change Log