CSS Masking Module Level 1

W3C Candidate Recommendation Draft,

This version:
https://www.w3.org/TR/2021/CRD-css-masking-1-20210805/
Latest published version:
https://www.w3.org/TR/css-masking-1/
Editor's Draft:
https://drafts.fxtf.org/css-masking-1/
Previous Versions:
Implementation Report:
https://test.csswg.org/harness/results/css-masking-1_dev/grouped/
Test Suite:
http://test.csswg.org/suites/css-masking/nightly-unstable/
Issue Tracking:
CSSWG Issues Repository
Inline In Spec
Editors:
(Adobe Inc.)
(Mozilla Japan)
Tab Atkins Jr. (Google)
Suggest an Edit for this Spec:
GitHub Editor

Abstract

CSS Masking provides two means for partially or fully hiding portions of visual elements: masking and clipping.

Masking describes how to use another graphical element or image as a luminance or alpha mask. Typically, rendering an element via CSS or SVG can conceptually be described as if the element, including its children, are drawn into a buffer and then that buffer is composited into the element’s parent. Luminance and alpha masks influence the transparency of this buffer before the compositing stage.

Clipping describes the visible region of visual elements. The region can be described by using certain SVG graphics elements or basic shapes. Anything outside of this region is not rendered.

CSS is a language for describing the rendering of structured documents (such as HTML and XML) on screen, on paper, etc.

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 https://www.w3.org/TR/.

This document was published by the CSS Working Group as a Candidate Recommendation Draft. Publication as a Candidate Recommendation does not imply endorsement by the W3C Membership. A Candidate Recommendation Draft integrates changes from the previous Candidate Recommendation that the Working Group intends to include in a subsequent Candidate Recommendation Snapshot.

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.

Please send feedback by filing issues in GitHub (preferred), including the spec code “css-masking” in the title, like this: “[css-masking] …summary of comment…”. All issues and comments are archived. Alternately, feedback can be sent to the (archived) public mailing list www-style@w3.org.

This document is governed by the 15 September 2020 W3C Process Document.

This document was produced by a group operating under the 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.

1. Introduction

This section is not normative.

This specification defines two different graphical operations which both fully or partly hide portions of an object: clipping and masking.

1.1. Clipping

A closed vector path, shape or polygon defines a so called clipping path. This clipping path is a region (in the absence of anti-aliasing) where everything on the “inside” of this region is allowed to show through but everything on the outside is “clipped out” and does not appear on the canvas.

Example Mask

A clipping path (middle) is applied on a polygon shaded with different colors (left). This results in a “clipped out” shape (right).

The clip-path property can use specified basic shapes as clipping path or reference an clipPath element with graphics elements to be used as clipping path.

1.2. Masking

The effect of applying a mask to a graphical object is as if the graphical object will be painted onto the background through a mask, thus completely or partially masking out parts of the graphical object.

Example Mask

A luminance mask (middle) is applied on a shape filled with a gradient (left). This results in a masked shape (right).

Masks are applied using the mask-image or mask-border-source properties.

The mask-image property may reference a mask element. The content of the mask element serves as the mask.

Alternatively, for many simple uses, the mask-image property may refer directly to images to be used as mask, forgoing the need for an explicit mask element. This mask can then be sized and positioned just like CSS background images using the mask-position, mask-size and other characterizing properties.

The mask-border-source property splits a mask into 9 pieces. The pieces may be sliced, scaled and stretched in various ways to fit the size of the mask border image area. The mask-border property serves as a shorthand property for mask-border-source and other characterizing properties.

The mask property serves as a shorthand property for all mask-border and mask-image affiliated properties.

Note: While masking gives many possibilities for enhanced graphical effects and in general provides more control over the “visible portions” of the content, clipping paths can perform better and basic shapes are easier to interpolate.

2. Module interactions

This specification defines a set of CSS properties that affect the visual rendering of elements to which those properties are applied. These effects are applied after elements have been sized and positioned according to the Visual formatting model from [CSS21]. Some values of these properties result in the creation of a stacking context. Furthermore, this specification replaces the section Clipping: the clip property from [CSS21].

The compositing model follows the SVG compositing model [SVG11]: First the element is styled under absence of filter effects, masking, clipping and opacity. Then the element and its descendants are drawn on a temporary canvas. In a last step the following effects are applied to the element in order: filter effects [FILTER-EFFECTS], clipping, masking and opacity.

This specification allows compositing multiple mask layers with the Porter Duff compositing operators defined in CSS Compositing and Blending [COMPOSITING-1].

The term object bounding box follows the definition in SVG 1.1 [SVG11].

3. Values

This specification follows the CSS property definition conventions from [CSS21]. Basic shapes are defined in CSS Shapes Module Level 1 [CSS-SHAPES]. Value types not defined in these specifications are defined in CSS Values and Units Module Level 3 [CSS3VAL].

In addition to the property-specific values listed in their definitions, all properties defined in this specification also accept CSS-wide keywords such as inherit as their property value [CSS3VAL]. For readability it has not been repeated explicitly.

4. Terminology

Definitions of CSS properties and values in this specification are analogous to definitions in CSS Backgrounds and Borders [CSS3BG]. To avoid redundancy, this specification relies on descriptions and definitions of CSS Backgrounds and Borders. The following terms in CSS Backgrounds and Borders have the following meaning in this specification:

Term in CSS Masking Term in [CSS3BG]
mask layer image background images
mask painting area background painting area
mask-size background-size
mask-position background-position
mask positioning area background positioning area
mask border image border-image
mask border image area border image area

5. Clipping Paths

The clipping path restricts the region to which paint can be applied, the so-called clipping region. Conceptually, any parts of the drawing that lie outside of this region are not drawn. This includes any content, background, borders, text decoration, outline and visible scrolling mechanism of the element to which the clipping path is applied, and those of its descendants.

An element’s ancestors may also clip portions of their content (e.g., via their own clip or clip-path properties and/or if their overflow property is not visible). What is rendered is the cumulative intersection.

If the clipping region exceeds the bounds of the UA’s document window, content may be clipped to that window by the native operating environment.

A clipping path affects the rendering of an element. It does not affect the element’s inherent geometry. The geometry of a clipped element (i.e. an element which references a clipPath element via a clip-path property, or a child of the referencing element) must remain the same as if it were not clipped.

Consider a shape that is clipped by a clipping path applied to an ancestor:
<g clip-path="circle()">
  <path id="shape" d="M0,0 L10,10, L 20,0 z"/>
</g>

The shape is referenced by a use element:

<use xlink:href="#shape"/>

The geometry of the shape is not influenced by the circular clipping path.

By default, pointer-events must not be dispatched on the clipped-out (non-visible) regions of a shape. For example, an element with a dimension of 10px to 10px which is clipped to a circle with a radius of 5px will not receive click events outside the clipping region.

5.1. Clipping Shape: the clip-path property

Name: clip-path
Value: <clip-source> | [ <basic-shape> || <geometry-box> ] | none
Initial: none
Applies to: All elements. In SVG, it applies to container elements excluding the defs element, all graphics elements and the use element
Inherited: no
Percentages: n/a
Computed value: as specified, but with <url> values made absolute
Canonical order: per grammar
Animation type: by computed value
Media: visual

Specifies a basic shape or references a clipPath element to create a clipping path.

<clip-source> = <url>
<geometry-box> = <shape-box> | fill-box | stroke-box | view-box
<basic-shape>

A basic shape function as defined in the CSS Shapes module [CSS-SHAPES]. A basic shape makes use of the specified reference box to size and position the basic shape. If no reference box is specified, the border-box will be used as reference box.

<geometry-box>

If specified in combination with a <basic-shape> it provides the reference box for the <basic-shape>.

If specified by itself, uses the edges of the specified box, including any corner shaping (e.g. defined by border-radius [CSS3BG]), as clipping path. See also “Shapes from box values” [CSS-SHAPES].

fill-box

Uses the object bounding box as reference box.

stroke-box

Uses the stroke bounding box as reference box.

view-box

Uses the nearest SVG viewport as reference box.

If a viewBox attribute is specified for the SVG viewport creating element:

  • The reference box is positioned at the origin of the coordinate system established by the viewBox attribute.

  • The dimension of the reference box is set to the width and height values of the viewBox attribute.

none

No clipping path gets created.

For SVG elements without associated CSS layout box, the used value for content-box and padding-box is fill-box and for border-box and margin-box is stroke-box.

For elements with associated CSS layout box, the used value for fill-box is content-box and for stroke-box and view-box is border-box.

A computed value of other than none results in the creation of a stacking context [CSS21] the same way that CSS opacity [CSS3COLOR] does for values other than 1.

If the URI reference is not valid (e.g it points to an object that doesn’t exist or the object is not a clipPath element), no clipping is applied.

This example demonstrates the use of the basic shape <polygon()> as clipping path. Each space separated length pair represents one point of the polygon. The visualized clipping path can be seen in the introduction.
clip-path: polygon(15px 99px, 30px 87px, 65px 99px, 85px 55px,
        122px 57px, 184px 73px, 198px 105px, 199px 150px,
        145px 159px, 155px 139px, 126px 120px, 112px 138px,
        80px 128px, 39px 126px, 24px 104px);
In this example, the clip-path property references an SVG clipPath element. Each comma separated length pair represents one point of the polygon. As for the previous example, the visualized clipping path can be seen in the introduction.

clip-path: url("#clip1");
<clipPath id="clip1">
    <polygon points="15,99 30,87 65,99 85,55 122,57 184,73 198,105
        199,150 145,159 155,139 126,120 112,138 80,128 39,126 24,104"/>
</clipPath>

The clip-path property is a presentation attribute for SVG elements.

6. SVG Clipping Path Sources

6.1. The clipPath element

Name: clipPath
Categories: container elements, never-rendered element
Content model: Any number of the following elements, in any order:
Attributes:
DOM Interfaces: SVGClipPathElement

Attribute definitions:

clipPathUnits = "userSpaceOnUse | objectBoundingBox"

Defines the coordinate system for the contents of the clipPath.

userSpaceOnUse

The contents of the clipPath represent values in the current user coordinate system in place at the time when the clipPath element is referenced (i.e., the user coordinate system for the element referencing the clipPath element via the clip-path property).

objectBoundingBox

The coordinate system has its origin at the top left corner of the bounding box of the element to which the clipping path applies to and the same width and height of this bounding box. User coordinates are sized equivalently to the CSS px unit.

If attribute clipPathUnits is not specified, then the effect is as if a value of userSpaceOnUse were specified.

Animatable: yes.

CSS properties inherit into the clipPath element from its ancestors; properties do not inherit from the element referencing the clipPath element.

clipPath elements are never rendered directly; their only usage is as something that can be referenced using the clip-path property. The display property does not apply to the clipPath element; thus, clipPath elements are not directly rendered even if the display property is set to a value other than none, and clipPath elements are available for referencing even when the display property on the clipPath element or any of its ancestors is set to none.

A clipPath element can contain path elements, text elements, basic shapes (such as circle) or a use element. If a use element is a child of a clipPath element, it must directly reference path, text or basic shapes elements. Indirect references are an error and the clipPath element must be ignored.

Firefox disables rendering of elements referencing clipPaths with violated content model. No browser ignores clipPath on use with indirect reference. <https://github.com/w3c/csswg-drafts/issues/17>

The raw geometry of each child element exclusive of rendering properties such as fill, stroke, stroke-width within a clipPath conceptually defines a 1-bit mask (with the possible exception of anti-aliasing along the edge of the geometry) which represents the silhouette of the graphics associated with that element. Anything outside the outline of the object is masked out. If a child element is made invisible by display or visibility it does not contribute to the clipping path. When the clipPath element contains multiple child elements, the silhouettes of the child elements are logically OR’d together to create a single silhouette which is then used to restrict the region onto which paint can be applied. Thus, a point is inside the clipping path if it is inside any of the children of the clipPath.

Define raw geometry with regards to CSS properties that affect it. Especially on text. <https://github.com/w3c/csswg-drafts/issues/170>

For a given graphics element, the actual clipping path used will be the intersection of the clipping path specified by its clip-path property (if any) with any clipping paths on its ancestors, as specified by the clip-path property on the elements which establish a new viewport. (See [SVG11])

A couple of additions:

6.2. Winding Rules: the clip-rule property

Name: clip-rule
Value: nonzero | evenodd
Initial: nonzero
Applies to: Applies to SVG graphics elements
Inherited: yes
Percentages: n/a
Computed value: as specified
Canonical order: per grammar
Animation type: discrete
Media: visual

The clip-rule property indicates the algorithm which is to be used to determine whether a given point is inside a shape for a clipping region created with a graphics element. The definition of the algorithms and the clip-rule values follows the definition of the fill-rule property. See section “Fill Properties” in SVG 1.1 [SVG11].

nonzero

See description of fill-rule property [SVG11].

evenodd

See description of fill-rule property [SVG11].

The clip-rule property only applies to graphics elements that are contained within a clipPath element.

Note: The clip-rule property does not apply to <basic-shape>s.

The following drawing illustrates the nonzero rule:

Shape with nonzero rule.

Three shapes from left to right: Star with 5 points drawn in one continuous, overlapping line; 2 clockwise drawn circles, one contains the other and both are subpaths of the same shape; 2 circles, one containing the other with the bigger one drawn in a clockwise direction and the smaller one in a counter-clockwise direction and both belonging to the same shape. Only the last shape has a "hole".

The following drawing illustrates the evenodd rule:

Shape with even-odd rule.

Three shapes from left to right: Star with 5 points drawn in one continuous, overlapping line; 2 clockwise drawn circles, one contains the other and both are subpaths of the same shape; 2 circles, one containing the other with the bigger one drawn in a clockwise direction and the smaller one in a counter-clockwise direction and both belonging to the same shape. All 3 shapes have a "hole".

The following fragment of code will cause an even-odd clipping rule to be applied to the clipping path because clip-rule is specified on the path element that defines the clipping shape:
<g clip-rule="nonzero">
  <clipPath id="MyClip">
    <path d="..." clip-rule="evenodd" />
  </clipPath>
  <rect clip-path="url(#MyClip)" ... />
</g>

whereas the following fragment of code will not cause an evenodd clipping rule to be applied because the clip-rule is specified on the referencing element, not on the object defining the clipping shape:

<g clip-rule="nonzero">
  <clipPath id="MyClip">
    <path d="..." />
  </clipPath>
  <rect clip-path="url(#MyClip)" clip-rule="evenodd" ... />
</g>

The clip-rule property is a presentation attribute for SVG elements.

7. Positioned Masks

7.1. Mask Image Source: the mask-image property

Name: mask-image
Value: <mask-reference>#
Initial: none
Applies to: All elements. In SVG, it applies to container elements excluding the defs element, all graphics elements and the use element
Inherited: no
Percentages: n/a
Computed value: the keyword none, a computed <image>, or a computed <url>
Canonical order: per grammar
Animation type: discrete
Media: visual

This property sets the mask layer image of an element. Where:

<mask-reference> = none | <image> | <mask-source>
<mask-source> = <url>
<url>

A URL reference to a mask element (for example url(commonmasks.svg#mask)) or to a CSS image.

none

A value of none counts as a transparent black image layer.

A computed value of other than none results in the creation of a stacking context [CSS21] the same way that CSS opacity [CSS3COLOR] does for values other than 1.

A mask reference that is an empty image (zero width or zero height), that fails to download, is not a reference to an mask element, is non-existent, or that cannot be displayed (e.g. because it is not in a supported image format) still counts as an image layer of transparent black.

See the section “Mask processing” for how to process a mask layer image.

Note: A value of none in a list of <mask-reference>s may influence the masking operation depending on the used compositing operator specified by mask-composite.

Note: A <mask-source> counts as mask layer and can be combined in a repeatable <mask-reference> list with <image> or further <mask-source> list items.

Note: An element can also be masked with mask-border-source. See mask-border-source for the interaction of that property with mask-image.

Examples for mask references:
body { mask-image: linear-gradient(black 0%, transparent 100%) }
p { mask-image: none }
div { mask-image: url(resources.svg#mask2) }

See the section “Layering multiple mask layer images” for how mask-image interacts with other comma-separated mask properties to form each mask layer.

7.2. Mask Image Interpretation: the mask-mode property

Name: mask-mode
Value: <masking-mode>#
Initial: match-source
Applies to: All elements. In SVG, it applies to container elements excluding the defs element, all graphics elements and the use element
Inherited: no
Percentages: n/a
Computed value: as specified
Canonical order: per grammar
Animation type: discrete
Media: visual

The mask-mode property indicates whether the <mask-reference> is treated as luminance mask or alpha mask. (See Mask processing.)

<masking-mode> = alpha | luminance | match-source

Values have the following meanings:

alpha

A value of alpha indicates that the alpha values of the mask layer image should be used as the mask values. See Calculating mask values.

luminance

A value of luminance indicates that the luminance values of the mask layer image should be used as the mask values. See Calculating mask values.

match-source

If the <mask-reference> of the mask-image property is of type <mask-source> the value specified by the referenced mask element’s mask-type property must be used.

If the <mask-reference> of the mask-image property is of type <image> the alpha values of the mask layer image should be used as the mask values.

In the following example, the mask-type property sets the mask type value for the mask element to alpha. The mask-image property has a reference to this mask element and the mask-mode property has a value of luminance. The mask-mode property will override the definition of mask-type to luminance.

The mask-mode property must not affect the masking mode of mask-border-source.

<mask id="SVGMask" mask-type="alpha" maskContentUnits="objectBoundingBox">
  <radialGradient id="radialFill">
    <stop stop-color="white" offset="0"/>
    <stop stop-color="black" offset="1"/>
  </radialGradient>
  <circle fill="url(#radialFill)" cx="0.5" cy="0.5" r="0.5"/>
</mask>

<style>
  rect {
    mask-image: url(#SVGMask);
    mask-mode: luminance;
  }
</style>

<rect width="200" height="200" fill="green"/>

See the section “Layering multiple mask layer images” for how mask-mode interacts with other comma-separated mask properties to form each mask layer.

7.3. Tiling Mask Images: the mask-repeat property

Name: mask-repeat
Value: <repeat-style>#
Initial: repeat
Applies to: All elements. In SVG, it applies to container elements excluding the defs element, all graphics elements and the use element
Inherited: no
Percentages: n/a
Computed value: Consists of: two keywords, one per dimension
Canonical order: per grammar
Animation type: discrete
Media: visual

Specifies how mask layer images are tiled after they have been sized and positioned.

See background-repeat property [CSS3BG] for the definitions of the property values.

body {
    background-color: blue;
    mask-image: url(dot-mask.png) luminance;
    mask-repeat: space;
}

Image of an element with a dotted mask.

The effect of space: the mask layer image of a dot is tiled to cover the whole mask painting area and the mask layer images are equally spaced.

See the section “Layering multiple mask layer images” for how mask-repeat interacts with other comma-separated mask properties to form each mask layer.

7.4. Positioning Mask Images: the mask-position property

Name: mask-position
Value: <position>#
Initial: 0% 0%
Applies to: All elements. In SVG, it applies to container elements excluding the defs element, all graphics elements and the use element
Inherited: no
Percentages: refer to size of mask painting area minus size of mask layer image; see text background-position [CSS3BG]
Computed value: Consisting of: two keywords representing the origin and two offsets from that origin, each given as an absolute length (if given a <length>), otherwise as a percentage.
Canonical order: per grammar
Animation type: repeatable list
Media: visual

See the background-position property [CSS3BG] for the definitions of the property values.

In the example below, the (single) image is placed in the lower-right corner of the viewport.
body {
    mask-image: url("logo.png");
    mask-position: 100% 100%;
    mask-repeat: no-repeat;
}
Mask positions can also be relative to other corners than the top left. E.g., the following puts the background image 10px from the bottom and 3em from the right:

mask-position: right 3em bottom 10px

See the section “Layering multiple mask layer images” for how mask-position interacts with other comma-separated mask properties to form each mask layer.

7.5. Masking Area: the mask-clip property

Name: mask-clip
Value: [ <geometry-box> | no-clip ]#
Initial: border-box
Applies to: All elements. In SVG, it applies to container elements excluding the defs element, all graphics elements and the use element
Inherited: no
Percentages: n/a
Computed value: as specified
Canonical order: per grammar
Animation type: discrete
Media: visual

For mask layer images that do not reference a mask element, mask-clip determines the mask painting area, which determines the area that is affected by the mask. The painted content of an element must be restricted to this area.

The mask-clip property has no affect on a mask layer image that references a mask element. The x, y, width, height and maskUnits attributes on the mask element determine the mask painting area for mask references.

Values have the following meanings:

content-box

The painted content is restricted to (clipped to) the content box.

padding-box

The painted content is restricted to (clipped to) the padding box.

border-box

The painted content is restricted to (clipped to) the border box.

fill-box

The painted content is restricted to (clipped to) the object bounding box.

stroke-box

The painted content is restricted to (clipped to) the stroke bounding box.

view-box

Uses the nearest SVG viewport as reference box.

If a viewBox attribute is specified for the SVG viewport creating element:

  • The reference box is positioned at the origin of the coordinate system established by the viewBox attribute.

  • The dimension of the reference box is set to the width and height values of the viewBox attribute.

no-clip

The painted content is not restricted (not clipped).

For SVG elements without associated CSS layout box, the used values for content-box and padding-box compute to fill-box and for border-box and margin-box compute to stroke-box.

For elements with associated CSS layout box, the used values for fill-box compute to content-box and for stroke-box and view-box compute to border-box.

See the section “Layering multiple mask layer images” for how mask-clip interacts with other comma-separated mask properties to form each mask layer.

7.6. Positioning Area: the mask-origin property

Name: mask-origin
Value: <geometry-box>#
Initial: border-box
Applies to: All elements. In SVG, it applies to container elements excluding the defs element, all graphics elements and the use element
Inherited: no
Percentages: n/a
Computed value: as specified
Canonical order: per grammar
Animation type: discrete
Media: visual

For elements rendered as a single box, specifies the mask positioning area. For elements rendered as multiple boxes (e.g., inline boxes on several lines, boxes on several pages) specifies which boxes box-decoration-break operates on to determine the mask positioning area.

content-box

The position is relative to the content box.

padding-box

The position is relative to the padding box. (For single boxes 0 0 is the upper left corner of the padding edge, 100% 100% is the lower right corner.)

border-box

The position is relative to the border box.

fill-box

The position is relative to the object bounding box.

stroke-box

The position is relative to the stroke bounding box.

view-box

Uses the nearest SVG viewport as reference box.

If a viewBox attribute is specified for the SVG viewport creating element:

  • The reference box is positioned at the origin of the coordinate system established by the viewBox attribute.

  • The dimension of the reference box is set to the width and height values of the viewBox attribute.

For SVG elements without associated CSS layout box, the values content-box, padding-box and border-box compute to fill-box.

For elements with associated CSS layout box, the values fill-box, stroke-box and view-box compute to the initial value of mask-origin.

Note: The mask-origin property is similar to the background-origin property [CSS3BG], but it has a different set of values, and a different initial value.

Note: If mask-clip is padding-box, mask-origin is border-box, mask-position is top left (the initial value), and the element has a non-zero border, then the top and left of the mask layer image will be clipped.

See the section “Layering multiple mask layer images” for how mask-origin interacts with other comma-separated mask properties to form each mask layer.

7.7. Sizing Mask Images: the mask-size property

Name: mask-size
Value: <bg-size>#
Initial: auto
Applies to: All elements. In SVG, it applies to container elements excluding the defs element, all graphics elements and the use element
Inherited: no
Percentages: n/a
Computed value: as specified, but with lengths made absolute
Canonical order: per grammar
Animation type: repeatable list
Media: visual

Specifies the size of the mask layer images.

See background-size property [CSS3BG] for the definitions of the property values.

See the section “Layering multiple mask layer images” for how mask-size interacts with other comma-separated mask properties to form each mask layer.

7.8. Compositing mask layers: the mask-composite property

Name: mask-composite
Value: <compositing-operator>#
Initial: add
Applies to: All elements. In SVG, it applies to container elements without the defs element and all graphics elements
Inherited: no
Percentages: n/a
Computed value: as specified
Canonical order: per grammar
Animation type: discrete
Media: visual
<compositing-operator> = add | subtract | intersect | exclude

Each keyword represents a Porter-Duff compositing operator [COMPOSITING-1] which defines the compositing operation used on the current mask layer with the mask layers below it.

In the following, the current mask layer is referred to source, all mask layers below it (with the corresponding compositing operators applied) are referred to destination.

add

The source is placed over the destination. (See Porter-Duff compositing operator source over for more details.)

subtract

The source is placed, where it falls outside of the destination. (See Porter-Duff compositing operator source out for more details.)

intersect

The parts of source that overlap the destination, replace the destination. (See Porter-Duff compositing operator source in .)

exclude

The non-overlapping regions of source and destination are combined. (See Porter-Duff compositing operator XOR.)

If there is no further mask layer, the compositing operator must be ignored. Mask layers must not composite with the element’s content or the content behind the element, instead they must act as if they are rendered into an isolated group.

All mask layers below the current mask layer must be composited before applying the compositing operation for the current mask layer.

This example uses two mask layer images: circle.svg and rect.svg.
Example of source-over compositing of mask layers

Both mask layer images are references with the mask-image property:

mask-image: circle.svg, rect.svg;

The mask layer with rect.svg is below the mask layer with circle.svg. That means circle.svg is closer to the user than rect.svg.

With the property mask-composite the author may choose different ways to combine multiple mask layers.

The following example specifies two mask layers and two compositing operators.
mask-image: rect.svg, circle.svg;
mask-composite: add, exclude;

rect.svg and circle.svg make use of the add compositing operator. There is no further mask layer to use exclude and therefore, exclude is ignored.

This is an example of 3 mask layers with different compositing operators.
mask-image: trapeze.svg, circle.svg, rect.svg;
mask-composite: subtract, add;

First, circle.svg is “added” to rect.svg. In a second step, only portions of trapeze.svg that are not overlapping the compositing result of the previous two layers is visible.

Example of source-over compositing of mask layers

See the section “Layering multiple mask layer images” for how mask-composite interacts with other comma-separated mask properties to form each mask layer.

7.9. Mask Shorthand: the mask property

Name: mask
Value: <mask-layer>#
Initial: see individual properties
Applies to: All elements. In SVG, it applies to container elements excluding the defs element, all graphics elements and the use element
Inherited: no
Percentages: see individual properties
Computed value: see individual properties
Canonical order: per grammar
Animation type: see individual properties
Media: visual
<mask-layer> = <mask-reference> || <position> [ / <bg-size> ]? ||<repeat-style> || <geometry-box> || [ <geometry-box> | no-clip ] || <compositing-operator> || <masking-mode>

If one <geometry-box> value and the no-clip keyword are present then <geometry-box> sets mask-origin and no-clip sets mask-clip to that value.

If one <geometry-box> value and no no-clip keyword are present then <geometry-box> sets both mask-origin and mask-clip to that value.

If two <geometry-box> values are present, then the first sets mask-origin and the second mask-clip.

The used value of the properties mask-repeat, mask-position, mask-clip, mask-origin and mask-size must have no effect if <mask-reference> references a mask element. In this case the element defines position, sizing and clipping of the mask layer image.

The mask shorthand also resets mask-border to its initial value. It is therefore recommended that authors use the mask shorthand, rather than other shorthands or the individual properties, to override any mask settings earlier in the cascade. This will ensure that mask-border has also been reset to allow the new styles to take effect.

7.10. The Mask Image Rendering Model

The application of the mask-image property with a value other than none to an element formatted with the CSS box model establishes a stacking context in the same way that CSS opacity [CSS3COLOR] does, and all the element’s descendants are rendered together as a group with the masking applied to the group as a whole.

The mask-image property has no effect on the geometry or hit-testing of any element’s CSS boxes.

The mask property is a presentation attribute for SVG elements.

7.10.1. Mask processing

In the following section, mask image refers either to a mask layer image or to a mask border image.

A mask image may be interpreted using one of two different methods with regards to calculating the mask values that will be multiplied with the target alpha values.

The first and simplest method of calculating the mask values is to use the alpha channel of the mask image. In this case the mask value at a given point is simply the value of the alpha channel at that point. The color channels do not contribute to the mask value.

The second method of calculating the mask values is to use the luminance of the mask image. In this case the mask value at a given point is computed from the color channel values and alpha channel value using the following procedure.

  1. Compute a luminance value from the color channel values.

    • If the computed value of color-interpolation on the mask element is linearRGB, convert the original image color values (potentially in the sRGB color space) to the linearRGB color space.

    • Then, using non-premultiplied RGB color values, apply the luminance-to-alpha coefficients (as defined in the feColorMatrix filter primitive [SVG11]) to convert the RGB color values to luminance values.

  2. Multiply the computed luminance value by the corresponding alpha value to produce the mask value.

Regardless of the method used, the procedure for calculating mask values assumes the content of the mask is a four-channel RGBA graphics object. For other types of graphics objects, special handling is required as follows.

For a three-channel RGB graphics object that is used in a mask (e.g., when referencing a three-channel image file), the effect is as if the object were converted into a four-channel RGBA image with the alpha channel uniformly set to 1.

For a single-channel image that is used in a mask (e.g., when referencing a single-channel grayscale image file), the effect is as if the object were converted into a four-channel RGBA image, where the single channel from the referenced object is used to compute the three color channels and the alpha channel is uniformly set to 1.

Note: When referencing a grayscale image file, the transfer curve relating the encoded grayscale values to linear light values must be taken into account when computing the color channels.

Note: SVG graphics elements (e.g., circle or text) are all treated as four-channel RGBA images for the purposes of masking operations.

The effect of a mask is identical to what would have happened if there were no mask but instead the alpha channel of the given object were multiplied with the mask’s resulting mask values.

Regions not covered by a mask image are treated as transparent black. The mask value is 0.

Note: Masks with repeating mask image tiles may have an offset to each other. The space between the mask images is treated as a transparent black mask.

7.10.2. Layering Multiple Mask Images

The mask of a box can have multiple layers. The number of layers is determined by the number of comma-separated values for the mask-image property. A value of none in a list of values with other <mask-reference>s still creates a layer.

See Layering Multiple Background Images [CSS3BG].

mask-mode and mask-composite do not have counterparts in CSS Backgrounds and Borders [CSS3BG]. Just like for the mask properties that do have a counterpart, the list of values are matched up from the first value: excess values at the end are not used. If a property doesn’t have enough comma-separated values to match the number of layers, the UA must calculate its used value by repeating the list of values until there are enough.

All mask layer images are transformed to alpha masks (if necessary see Mask processing) and combined by compositing taking the compositing operators specified by mask-composite into account.

8. Border-Box Mask

With mask-border an image can be split into nine pieces: four corners, four edges and the middle piece as demonstrated in the figure below.

pieces of a mask border image

Pieces of a mask border image.

These pieces may be sliced, scaled and stretched in various ways to fit the size of the mask border image area. This distorted image is then used as a mask. The syntax of mask-border corresponds to the border-image property of CSS Background and Borders [CSS3BG].

The mask border image in the following example is split into four corners with dimensions of 75 pixels, four edges and the middle piece that is stretched and scaled.
Example for 'mask-border'

Example for mask-border. The object on the left is the object to mask. The second image is the alpha mask and the last image the masked object.

div {
    background: linear-gradient(bottom, #F27BAA 0%, #FCC8AD 100%);
    mask-border-slice: 25 fill;
    mask-border-repeat: stretch;
    mask-border-source: url(mask.png);
}

8.1. Mask Border Image Source: the mask-border-source property

Name: mask-border-source
Value: none | <image>
Initial: none
Applies to: All elements. In SVG, it applies to container elements excluding the defs element, all graphics elements and the use element
Inherited: no
Percentages: n/a
Computed value: they keyword none or the computed <image>
Canonical order: per grammar
Animation type: discrete
Media: visual

Specifies an image to be used as mask border image.

An image that is an empty image (zero width or zero height), that fails to download, is non-existent, or that cannot be displayed (e.g. because it is not in a supported image format) is ignored. It still counts as an mask border image but does not mask the element.

See “Mask processing” on how to process the mask border image.

A computed value of other than none results in the creation of a stacking context [CSS21] the same way that CSS opacity [CSS3COLOR] does for values other than 1.

mask-border-source and mask-image can be specified independent of each other. If both properties have a value other than none, the element is masked by both masking operations one after the other.

Note: It does not matter if mask-image is applied to the element before or after mask-border-source. Both operation orders result in the same rendering.

8.2. Mask Border Image Interpretation: the mask-border-mode property

Name: mask-border-mode
Value: luminance | alpha
Initial: alpha
Applies to: All elements. In SVG, it applies to container elements excluding the defs element, all graphics elements and the use element
Inherited: no
Percentages: n/a
Computed value: as specified
Canonical order: per grammar
Animation type: discrete
Media: visual

The mask-border-mode property indicates whether the <image> value for mask-border-source is treated as luminance mask or alpha mask. (See Mask processing.)

Values have the following meanings:

alpha

A value of alpha indicates that the alpha values of the mask border image should be used as the mask values. See Calculating mask values.

luminance

A value of luminance indicates that the luminance values of the mask border image should be used as the mask values. See Calculating mask values.

The mask-mode and mask-type properties must have no affect on the mask border image type.

8.3. Mask Border Image Slicing: the mask-border-slice property

Name: mask-border-slice
Value: <number-percentage>{1,4} fill?
Initial: 0
Applies to: All elements. In SVG, it applies to container elements excluding the defs element, all graphics elements and the use element
Inherited: no
Percentages: refer to size of the mask border image
Computed value: as specified
Canonical order: per grammar
Animation type: discrete
Media: visual

This property specifies inward offsets from the top, right, bottom, and left edges of the mask border image, dividing it into nine regions: four corners, four edges and a middle. The middle image part is discarded and treated as fully opaque white (the content covered by the middle part is not masked and shines through) unless the fill keyword is present.

See the border-image-slice property [CSS3BG] for the definitions of the property values.

8.4. Masking Areas: the mask-border-width property

Name: mask-border-width
Value: [ <length-percentage> | <number> | auto ]{1,4}
Initial: auto
Applies to: All elements. In SVG, it applies to container elements excluding the defs element, all graphics elements and the use element
Inherited: no
Percentages: relative to width/height of the mask border image area
Computed value: all <length>s made absolute, otherwise as specified
Canonical order: per grammar
Animation type: discrete
Media: visual

The mask border image is drawn inside an area called the mask border image area. This is an area whose boundaries by default correspond to the border box, see mask-border-outset.

See the border-image-width property [CSS3BG] for the definitions of the property values.

Note: For SVG elements without an associated layout box the border-width is considered to be 0.

8.5. Edge Overhang: the mask-border-outset property

Name: mask-border-outset
Value: [ <length> | <number> ]{1,4}
Initial: 0
Applies to: All elements. In SVG, it applies to container elements excluding the defs element, all graphics elements and the use element
Inherited: no
Percentages: n/a
Computed value: all <length>s made absolute, otherwise as specified
Canonical order: per grammar
Animation type: discrete
Media: visual

The values specify the amount by which the mask border image area extends beyond the border box. If it has four values, they set the outsets on the top, right, bottom and left sides in that order. If the left is missing, it is the same as the right; if the bottom is missing, it is the same as the top; if the right is missing, it is the same as the top.

As with mask-border-width, a <number> represents a multiple of the corresponding border-width. Negative values are not allowed for any of the mask-border-outset values.

Note: For SVG elements without associated layout box the border-width is considered to be 0.

8.6. Mask Border Image Tiling: the mask-border-repeat property

Name: mask-border-repeat
Value: [ stretch | repeat | round | space ]{1,2}
Initial: stretch
Applies to: All elements. In SVG, it applies to container elements excluding the defs element, all graphics elements and the use element
Inherited: no
Percentages: n/a
Computed value: as specified
Canonical order: per grammar
Animation type: discrete
Media: visual

This property specifies how the images for the sides and the middle part of the mask border image are scaled and tiled. The first keyword applies to the horizontal sides, the second to the vertical ones. If the second keyword is absent, it is assumed to be the same as the first.

See the border-image-repeat property [CSS3BG] for the definitions of the property values.

The exact process for scaling and tiling the mask border image parts is given in the section Masking with the mask border image

8.7. Mask Border Image Shorthand: the mask-border property

Name: mask-border
Value: <'mask-border-source'> || <'mask-border-slice'> [ / <'mask-border-width'>? [ / <'mask-border-outset'> ]? ]? || <'mask-border-repeat'> || <'mask-border-mode'>
Initial: See individual properties
Applies to: See individual properties
Inherited: no
Percentages: n/a
Computed value: See individual properties
Canonical order: per grammar
Animation type: See individual properties
Media: visual

This is a shorthand property for setting mask-border-source, mask-border-slice, mask-border-width, mask-border-outset, mask-border-repeat and mask-border-mode. Omitted values are set to their initial values.

Note: The mask shorthand resets the properties mask-border, mask-border-source, mask-border-slice, mask-border-width, mask-border-outset, mask-border-repeat and mask-border-mode.

8.8. Masking with the mask border image

After the mask border image given by mask-border-source is sliced by the mask-border-slice values, the resulting nine images are scaled, positioned, and tiled into their corresponding mask border image regions in four steps as described in the section Drawing the Border Image [CSS3BG].

The application of the mask-border-source property to an element formatted with the CSS box model establishes a stacking context in the same way that CSS opacity [CSS3COLOR] does, and all the element’s descendants are rendered together as a group with the masking applied to the group as a whole.

The mask-border-source property has no effect on the geometry or hit-testing of any element’s CSS boxes.

9. SVG Mask Sources

9.1. The mask element

Name: mask
Categories: container elements, never-rendered element
Content model: Any number of the following elements, in any order:
Attributes:
DOM Interfaces: SVGMaskElement

Attribute definitions:

maskUnits = "userSpaceOnUse | objectBoundingBox"

Defines the coordinate system for attributes x, y, width and height.

userSpaceOnUse

x, y, width and height represent values in the current user coordinate system [CSS3-TRANSFORMS] in place at the time when the mask element is referenced (i.e., the user coordinate system for the element referencing the mask element via the mask property).

objectBoundingBox

x, y, width and height represent fractions or percentages of the object bounding box of the element to which the mask is applied. User coordinates are sized equivalently to the CSS px unit.

If attribute maskUnits is not specified, then the effect is as if a value of objectBoundingBox were specified.

Animatable: yes.

maskContentUnits = "userSpaceOnUse | objectBoundingBox"

Defines the coordinate system for the contents of the mask.

userSpaceOnUse

The user coordinate system for the contents of the mask element is the current user coordinate system in place at the time when the mask element is referenced (i.e., the user coordinate system for the element referencing the mask element via the mask property).

objectBoundingBox

The coordinate system has its origin at the top left corner of the bounding box of the element to which the clipping path applies to and the same width and height of this bounding box. User coordinates are sized equivalently to the CSS px unit.

If attribute maskContentUnits is not specified, then the effect is as if a value of userSpaceOnUse were specified.

Animatable: yes.

x = "<length-percentage>"

The x-axis coordinate of one corner of the rectangle for the largest possible offscreen buffer. If the attribute is not specified but at least one of the attributes y, width or height are specified, the effect is as if a value of -10% were specified.

Animatable: yes.

y = "<length-percentage>"

The y-axis coordinate of one corner of the rectangle for the largest possible offscreen buffer. If the attribute is not specified but at least one of the attributes x, width or height are specified, the effect is as if a value of -10% were specified.

Animatable: yes.

width = "<length-percentage>"

The width of the largest possible offscreen buffer. A negative value or a value of zero disables rendering of the element. If the attribute is not specified but at least one of the attributes x, y or height are specified, the effect is as if a value of 120% were specified.

Animatable: yes.

height = "<length-percentage>"

The height of the largest possible offscreen buffer. A negative value or a value of zero disables rendering of the element. If the attribute is not specified but at least one of the attributes x, y or width are specified, the effect is as if a value of 120% were specified.

Animatable: yes.

If at least one of the attributes x, y, width or height are specified, the given object and the rectangle defined by x, y, width and height establish a current clipping path. The rendered content of the mask must be clipped by this current clipping path.

CSS properties inherit into the mask element from its ancestors; properties do not inherit from the element referencing the mask element.

mask elements are never rendered directly; their only usage is as something that can be referenced using the mask property. The opacity, filter and display properties do not apply to the mask element; thus, mask elements are not directly rendered even if the display property is set to a value other than none, and mask elements are available for referencing even when the display property on the mask element or any of its ancestors is set to none.

9.2. Mask Source Interpretation: the mask-type property

Name: mask-type
Value: luminance | alpha
Initial: luminance
Applies to: mask elements
Inherited: no
Percentages: n/a
Computed value: as specified
Canonical order: per grammar
Animation type: discrete
Media: visual

The mask-type property defines whether the content of the mask element is treated as as luminance mask or alpha mask, as described in Calculating mask values.

Values have the following meanings:

luminance

Indicates that the luminance values of the mask should be used.

alpha

Indicates that the alpha values of the mask should be used.

The mask-type property allows the author of the mask element to specify the preferred masking mode. However, the author can override this preference by setting the mask-mode value to something different than match-source on the masked content.

In the following example the computed value of mask-type is luminance and the computed value of mask-mode is match-source. The UA must follow the preferred masking mode defined on the mask element.
<svg>
  <mask style="mask-type: luminance;" id="mask">
    ...
  </mask>
</svg>

<p style="mask-image: url(#mask); mask-mode: auto;">
  This is the masked content.
</p>

In the next example the computed value of mask-mode is alpha and overrides the preference on the mask element that is computed to luminance. The mask layer image is used as an alpha mask.

lt;svg>
 <mask style="mask-type: luminance;" id="mask2">
   ...
 </mask>
lt;/svg>

lt;p style="mask-image: url(#mask2); mask-mode: alpha;">
 This is the masked content.
lt;/p>

The mask-type property is a presentation attribute for SVG elements.

10. Privacy and Security Considerations

It is important that the timing to the masking operations is independent of the source and destination pixel. Masking operations must be implemented in such a way that they always take the same amount of time regardless of the pixel values. If this rule is not followed, an attacker could infer information and mount a timing attack.

A timing attack is a method of obtaining information about content that is otherwise protected, based on studying the amount of time it takes for an operation to occur. If, for example, red pixels took longer to draw than green pixels, one might be able to reconstruct a rough image of the element being rendered, without ever having access to the content of the element.

<mask-source>s and <clip-source>s have special requirements on fetching resources.

User agents must use the potentially CORS-enabled fetch method defined by the [FETCH] specification for all <mask-source>, <clip-source> and <image> values on the mask-image, mask-border-source and clip-path properties. When fetching, user agents must use “Anonymous” mode, set the referrer source to the stylesheet’s URL and set the origin to the URL of the containing document. If this results in network errors, the effect is as if the value none had been specified.

Appendix A: The deprecated clip property

Name: clip
Value: rect() | auto
Initial: auto
Applies to: Absolutely positioned elements. In SVG, it applies to elements which establish a new viewport, pattern elements and mask elements.
Inherited: no
Percentages: n/a
Computed value: as specified
Canonical order: per grammar
Animation type: by computed value
Media: visual

With this specification the clip property is deprecated. Authors are encouraged to use the clip-path property instead. UAs must support the clip property.

The clip property applies only to absolutely positioned elements. In SVG, it applies to elements which establish a new viewport, pattern elements and mask elements. Values have the following meanings:

auto

The element does not clip.

rect() = rect( <top>, <right>, <bottom>, <left> )

<top> and <bottom> specify offsets from the top border edge of the box, and <right>, and <left> specify offsets from the left border edge of the box. Authors should separate offset values with commas. User agents must support separation with commas, but may also support separation without commas (but not a combination), because a previous revision of this specification was ambiguous in this respect.

<top>, <right>, <bottom>, and <left> may either have a <length> value or auto. Negative lengths are permitted. The value auto means that a given edge of the clipping region will be the same as the edge of the element’s generated border box (i.e., auto means the same as 0 for <top> and <left>, the same as the used value of the height plus the sum of vertical padding and border widths for <bottom>, and the same as the used value of the width plus the sum of the horizontal padding and border widths for <right>, such that four auto values result in the clipping region being the same as the element’s border box).

When coordinates are rounded to pixel coordinates, care should be taken that no pixels remain visible when <left> and <right> have the same value (or <top> and <bottom> have the same value), and conversely that no pixels within the element’s border box remain hidden when these values are auto.

The clip property is a presentation attribute for SVG elements.

Example: The following two rules:
p#one { clip: rect(5px, 40px, 45px, 5px); }
p#two { clip: rect(5px, 55px, 45px, 5px); }

and assuming both Ps are 50 by 55 pixel, will create, respectively, the rectangular clipping regions delimited by the dashed lines in the following illustrations:

Values for rect shape

This diagram illustrates two block boxes, one next to the other, with rectangular clipping regions of different dimensions. (See long description.)

Appendix B: Compute stroke bounding box

The algorithm to compute the stroke bounding box is as follows, depending on the type of element:

a graphics element without use or image
an a element with a text content element
  1. Let box be a rectangle initialized to the object bounding box of element.

  2. If the used value of stroke-width <= 0 or the used value of stroke is none return box.

  3. Let delta be the inflation value initialized to the half of the stroke-width.

  4. If element is not rect, ellipse, circle or image just follow one of the following conditions in the order they apply:

    the used value for stroke-linejoin is miter
    1. Let miter be the used value of stroke-miterlimit.

    2. If miter is smaller than the square root of 2 and if the used value for stroke-linecap is square, multiply delta with the square root of 2. Otherwise, multiply delta with miter.

    the used value for stroke-linecap is square
    1. Multiply delta with the square root of 2.

  5. Inflate box with the value of delta.

  6. Return box.

Note: The values of the stroke-opacity, stroke-dasharray and stroke-dashoffset do not affect the calculation of the stroke bounding box.

a container element
use
  1. Let parent be the container element if it is one, or the root of the use element’s shadow tree otherwise.

  2. For each child child of parent

    1. Invoke the stroke bounding box algorithm with child.

    2. Let childBox be the returned box value of the invoked algorithm.

    3. Map childBox from the coordinate space of child to the coordinate space of parent.

  3. Let box be the union of all childBoxes.

  4. Return box.

image
  1. Return the object bounding box of element.

Note: A future version of the SVG specification may override this section.

Appendix C: DOM interfaces

Interface SVGClipPathElement

The SVGClipPathElement interface corresponds to the clipPath element.

[Exposed=Window]
interface SVGClipPathElement : SVGElement {
  readonly attribute SVGAnimatedEnumeration clipPathUnits;
  readonly attribute SVGAnimatedTransformList transform;
};
Attributes:
clipPathUnits, of type SVGAnimatedEnumeration, readonly

Corresponds to attribute clipPathUnits on the given clipPath element. Takes one of the constants defined in SVGUnitTypes.

transform, of type SVGAnimatedTransformList, readonly

Corresponds to presentation attribute transform on the given element.

Interface SVGMaskElement

The SVGMaskElement interface corresponds to the mask element.

[Exposed=Window]
interface SVGMaskElement : SVGElement {
  readonly attribute SVGAnimatedEnumeration maskUnits;
  readonly attribute SVGAnimatedEnumeration maskContentUnits;
  readonly attribute SVGAnimatedLength x;
  readonly attribute SVGAnimatedLength y;
  readonly attribute SVGAnimatedLength width;
  readonly attribute SVGAnimatedLength height;
};
Attributes:
maskUnits, of type SVGAnimatedEnumeration, readonly

Corresponds to attribute maskUnits on the given mask element. Takes one of the constants defined in SVGUnitTypes.

maskContentUnits, of type SVGAnimatedEnumeration, readonly

Corresponds to attribute maskContentUnits on the given mask element. Takes one of the constants defined in SVGUnitTypes.

x, of type SVGAnimatedLength, readonly

Corresponds to attribute x on the given mask element.

y, of type SVGAnimatedLength, readonly

Corresponds to attribute y on the given mask element.

width, of type SVGAnimatedLength, readonly

Corresponds to attribute width on the given mask element.

height, of type SVGAnimatedLength, readonly

Corresponds to attribute height on the given mask element.

Changes since last publication

The following changes were made since the 26 August 2014 Candidate Recommendation.

The following changes were made since the 22 May 2014 Working Draft.

The following changes were made since the 13 February 2014 Working Draft.

The following changes were made since the 29 October 2013 Last Call Working Draft.

The following significant changes were made since the 20 June 2013 Working Draft.

The following significant changes were made since the 15 November 2012 Working Draft.

See detailed list of changes in the ChangeLog.

Acknowledgments

Thanks to Elika J. Etemad, Cameron McCormack, Liam R. E. Quin, Björn Höhrmann, Alan Stearns, Jarek Foksa, David Baron, Boris Zbarsky, Markus Stange and Sara Soueidan for their careful reviews, comments, and corrections. Special thanks to CJ Gammon for graphical assets.

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.

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

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.

Implementations of Unstable and Proprietary Features

To avoid clashes with future stable CSS features, the CSSWG recommends following best practices for the implementation of unstable features and proprietary extensions to CSS.

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 https://www.w3.org/Style/CSS/Test/. Questions should be directed to the public-css-testsuite@w3.org mailing list.

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.

Index

Terms defined by this specification

Terms defined by reference

References

Normative References

[COMPOSITING-1]
Rik Cabanier; Nikos Andronikos. Compositing and Blending Level 1. 13 January 2015. CR. URL: https://www.w3.org/TR/compositing-1/
[CSS-BREAK-4]
Rossen Atanassov; Elika Etemad. CSS Fragmentation Module Level 4. 18 December 2018. WD. URL: https://www.w3.org/TR/css-break-4/
[CSS-CASCADE-5]
Elika Etemad; Miriam Suzanne; Tab Atkins Jr.. CSS Cascading and Inheritance Level 5. 8 June 2021. WD. URL: https://www.w3.org/TR/css-cascade-5/
[CSS-COLOR-4]
Tab Atkins Jr.; Chris Lilley. CSS Color Module Level 4. 1 June 2021. WD. URL: https://www.w3.org/TR/css-color-4/
[CSS-DISPLAY-3]
Tab Atkins Jr.; Elika Etemad. CSS Display Module Level 3. 18 December 2020. CR. URL: https://www.w3.org/TR/css-display-3/
[CSS-FONTS-4]
John Daggett; Myles Maxfield; Chris Lilley. CSS Fonts Module Level 4. 29 July 2021. WD. URL: https://www.w3.org/TR/css-fonts-4/
[CSS-FONTS-5]
Myles Maxfield; Chris Lilley. CSS Fonts Module Level 5. 29 July 2021. WD. URL: https://www.w3.org/TR/css-fonts-5/
[CSS-IMAGES-3]
Tab Atkins Jr.; Elika Etemad; Lea Verou. CSS Images Module Level 3. 17 December 2020. CR. URL: https://www.w3.org/TR/css-images-3/
[CSS-INLINE-3]
Dave Cramer; Elika Etemad; Steve Zilles. CSS Inline Layout Module Level 3. 27 August 2020. WD. URL: https://www.w3.org/TR/css-inline-3/
[CSS-OVERFLOW-3]
David Baron; Elika Etemad; Florian Rivoal. CSS Overflow Module Level 3. 3 June 2020. WD. URL: https://www.w3.org/TR/css-overflow-3/
[CSS-SHAPES]
Vincent Hardy; Rossen Atanassov; Alan Stearns. CSS Shapes Module Level 1. 20 March 2014. CR. URL: https://www.w3.org/TR/css-shapes-1/
[CSS-TEXT-3]
Elika Etemad; Koji Ishii; Florian Rivoal. CSS Text Module Level 3. 22 April 2021. CR. URL: https://www.w3.org/TR/css-text-3/
[CSS-TEXT-DECOR-3]
Elika Etemad; Koji Ishii. CSS Text Decoration Module Level 3. 13 August 2019. CR. URL: https://www.w3.org/TR/css-text-decor-3/
[CSS-UI-3]
Tantek Çelik; Florian Rivoal. CSS Basic User Interface Module Level 3 (CSS3 UI). 21 June 2018. REC. URL: https://www.w3.org/TR/css-ui-3/
[CSS-VALUES-4]
Tab Atkins Jr.; Elika Etemad. CSS Values and Units Module Level 4. 15 July 2021. WD. URL: https://www.w3.org/TR/css-values-4/
[CSS-WRITING-MODES-3]
Elika Etemad; Koji Ishii. CSS Writing Modes Level 3. 10 December 2019. REC. URL: https://www.w3.org/TR/css-writing-modes-3/
[CSS-WRITING-MODES-4]
Elika Etemad; Koji Ishii. CSS Writing Modes Level 4. 30 July 2019. CR. URL: https://www.w3.org/TR/css-writing-modes-4/
[CSS21]
Bert Bos; et al. Cascading Style Sheets Level 2 Revision 1 (CSS 2.1) Specification. 7 June 2011. REC. URL: https://www.w3.org/TR/CSS21/
[CSS3-TRANSFORMS]
Simon Fraser; et al. CSS Transforms Module Level 1. 14 February 2019. CR. URL: https://www.w3.org/TR/css-transforms-1/
[CSS3BG]
Bert Bos; Elika Etemad; Brad Kemper. CSS Backgrounds and Borders Module Level 3. 26 July 2021. CR. URL: https://www.w3.org/TR/css-backgrounds-3/
[CSS3VAL]
Tab Atkins Jr.; Elika Etemad. CSS Values and Units Module Level 3. 6 June 2019. CR. URL: https://www.w3.org/TR/css-values-3/
[FETCH]
Anne van Kesteren. Fetch Standard. Living Standard. URL: https://fetch.spec.whatwg.org/
[FILL-STROKE-3]
Elika Etemad; Tab Atkins Jr.. CSS Fill and Stroke Module Level 3. 13 April 2017. WD. URL: https://www.w3.org/TR/fill-stroke-3/
[FILTER-EFFECTS]
Dirk Schulze; Dean Jackson. Filter Effects Module Level 1. 18 December 2018. WD. URL: https://www.w3.org/TR/filter-effects-1/
[RFC2119]
S. Bradner. Key words for use in RFCs to Indicate Requirement Levels. March 1997. Best Current Practice. URL: https://datatracker.ietf.org/doc/html/rfc2119
[SVG11]
Erik Dahlström; et al. Scalable Vector Graphics (SVG) 1.1 (Second Edition). 16 August 2011. REC. URL: https://www.w3.org/TR/SVG11/
[SVG2]
Amelia Bellamy-Royds; et al. Scalable Vector Graphics (SVG) 2. 4 October 2018. CR. URL: https://www.w3.org/TR/SVG2/
[WebIDL]
Boris Zbarsky. Web IDL. 15 December 2016. ED. URL: https://heycam.github.io/webidl/

Informative References

[CSS3COLOR]
Tantek Çelik; Chris Lilley; David Baron. CSS Color Module Level 3. 19 June 2018. REC. URL: https://www.w3.org/TR/css-color-3/

Property Index

Name Value Initial Applies to Inh. %ages Anim­ation type Canonical order Com­puted value Media
clip rect() | auto auto Absolutely positioned elements. In SVG, it applies to elements which establish a new viewport, pattern elements and mask elements. no n/a by computed value per grammar as specified visual
clip-path <clip-source> | [ <basic-shape> || <geometry-box> ] | none none All elements. In SVG, it applies to container elements excluding the defs element, all graphics elements and the use element no n/a by computed value per grammar as specified, but with <url> values made absolute visual
clip-rule nonzero | evenodd nonzero Applies to SVG graphics elements yes n/a discrete per grammar as specified visual
mask <mask-layer># see individual properties All elements. In SVG, it applies to container elements excluding the defs element, all graphics elements and the use element no see individual properties see individual properties per grammar see individual properties visual
mask-border <'mask-border-source'> || <'mask-border-slice'> [ / <'mask-border-width'>? [ / <'mask-border-outset'> ]? ]? || <'mask-border-repeat'> || <'mask-border-mode'> See individual properties See individual properties no n/a See individual properties per grammar See individual properties visual
mask-border-mode luminance | alpha alpha All elements. In SVG, it applies to container elements excluding the defs element, all graphics elements and the use element no n/a discrete per grammar as specified visual
mask-border-outset [ <length> | <number> ]{1,4} 0 All elements. In SVG, it applies to container elements excluding the defs element, all graphics elements and the use element no n/a discrete per grammar all <length>s made absolute, otherwise as specified visual
mask-border-repeat [ stretch | repeat | round | space ]{1,2} stretch All elements. In SVG, it applies to container elements excluding the defs element, all graphics elements and the use element no n/a discrete per grammar as specified visual
mask-border-slice <number-percentage>{1,4} fill? 0 All elements. In SVG, it applies to container elements excluding the defs element, all graphics elements and the use element no refer to size of the mask border image discrete per grammar as specified visual
mask-border-source none | <image> none All elements. In SVG, it applies to container elements excluding the defs element, all graphics elements and the use element no n/a discrete per grammar they keyword none or the computed <image> visual
mask-border-width [ <length-percentage> | <number> | auto ]{1,4} auto All elements. In SVG, it applies to container elements excluding the defs element, all graphics elements and the use element no relative to width/height of the mask border image area discrete per grammar all <length>s made absolute, otherwise as specified visual
mask-clip [ <geometry-box> | no-clip ]# border-box All elements. In SVG, it applies to container elements excluding the defs element, all graphics elements and the use element no n/a discrete per grammar as specified visual
mask-composite <compositing-operator># add All elements. In SVG, it applies to container elements without the defs element and all graphics elements no n/a discrete per grammar as specified visual
mask-image <mask-reference># none All elements. In SVG, it applies to container elements excluding the defs element, all graphics elements and the use element no n/a discrete per grammar the keyword none, a computed <image>, or a computed <url> visual
mask-mode <masking-mode># match-source All elements. In SVG, it applies to container elements excluding the defs element, all graphics elements and the use element no n/a discrete per grammar as specified visual
mask-origin <geometry-box># border-box All elements. In SVG, it applies to container elements excluding the defs element, all graphics elements and the use element no n/a discrete per grammar as specified visual
mask-position <position># 0% 0% All elements. In SVG, it applies to container elements excluding the defs element, all graphics elements and the use element no refer to size of mask painting area minus size of mask layer image; see text background-position [CSS3BG] repeatable list per grammar Consisting of: two keywords representing the origin and two offsets from that origin, each given as an absolute length (if given a <length>), otherwise as a percentage. visual
mask-repeat <repeat-style># repeat All elements. In SVG, it applies to container elements excluding the defs element, all graphics elements and the use element no n/a discrete per grammar Consists of: two keywords, one per dimension visual
mask-size <bg-size># auto All elements. In SVG, it applies to container elements excluding the defs element, all graphics elements and the use element no n/a repeatable list per grammar as specified, but with lengths made absolute visual
mask-type luminance | alpha luminance mask elements no n/a discrete per grammar as specified visual

IDL Index

[Exposed=Window]
interface SVGClipPathElement : SVGElement {
  readonly attribute SVGAnimatedEnumeration clipPathUnits;
  readonly attribute SVGAnimatedTransformList transform;
};

[Exposed=Window]
interface SVGMaskElement : SVGElement {
  readonly attribute SVGAnimatedEnumeration maskUnits;
  readonly attribute SVGAnimatedEnumeration maskContentUnits;
  readonly attribute SVGAnimatedLength x;
  readonly attribute SVGAnimatedLength y;
  readonly attribute SVGAnimatedLength width;
  readonly attribute SVGAnimatedLength height;
};

Issues Index

Firefox disables rendering of elements referencing clipPaths with violated content model. No browser ignores clipPath on use with indirect reference. <https://github.com/w3c/csswg-drafts/issues/17>
Define raw geometry with regards to CSS properties that affect it. Especially on text. <https://github.com/w3c/csswg-drafts/issues/170>