CSS Masking Module Level 1

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 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, in speech, etc.

Status of this document

Table of contents

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

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.

The clip-path property can reference an SVG graphics element or use a specified basic shapes as clipping path.

1.2 Masking

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

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

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

The mask-image property may reference a mask element. In this case the content of the mask element is rendered into temporary canvas which has been initialized to transparent black. This temporary canvas is taken as mask image. Alternatively, for many simple uses, the mask-image property may refer directly to images to be used as mask image, forgoing the need for an explicit mask element. The mask property serves as a shorthand property for mask-box, mask-image and other characterizing properties.

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

The terms object bounding box and stroke bounding box follow the definitions in SVG 2 [SVG2].

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 the inherit keyword as their property value. 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:

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 is conceptually equivalent to a custom viewport for the element it applies to. Thus, it affects the rendering of an element. It does not affects the element’s inherent geometry. The boundaries 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.

<g clip-path="circle()">
  <path id="shape" d="M0,0 L10,10, L 20,0 z"/>
</g>

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

By default, pointer events must not be dispatched on the clipped (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

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 values content-box, padding-box, border-box and margin-box compute to fill-box.

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

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.

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);

clip-path: url("#clip1");

<svg>
<clipPath id="clip1">
  <path d="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>
</svg>

6 SVG Clipping Path Sources

6.1 The clipPath element

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.

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.

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:

• The clipPath element itself and its child elements do not inherit clipping paths from the ancestors of the clipPath element.
• The clipPath element or any of its children can specify property clip-path.
  If a valid clip-path reference is placed on a clipPath element, the resulting clipping path is the intersection of the contents of the clipPath element with the referenced clipping path.
  If a valid clip-path reference is placed on one of the children of a clipPath element, then the given child element is clipped by the referenced clipping path before OR’ing the silhouette of the child element with the silhouettes of the other child elements.
• An empty clipping path will completely clip away the element that had the clip-path property applied.

6.2 Winding Rules: the clip-rule property

The clip-rule property specifies the used winding rule for a graphics element.

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.

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

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

7 Positioned Masks

7.1 Mask Image Source: the mask-image property

This property sets the mask 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 an image layer but does not mask the element.

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

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) is ignored. It still counts as an image layer but does not mask the element.

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

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

7.2 Mask Image Interpretation: the mask-type property

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

<source-type> = alpha | luminance | auto

Values have the following meanings:

alpha

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

auto

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

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

<mask id="SVGMask" mask-source-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-type: luminance;
  }
</style>

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

7.3 Tiling Mask Images: The mask-repeat property

Specifies how mask 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;
}

7.4 Positioning Mask Images: the mask-position property

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

body {
  mask-image: url("logo.png");
  mask-position: 100% 100%;
  mask-repeat: no-repeat;
}

mask-position: right 3em bottom 10px

7.5 Masking Area: the mask-clip property

Determines the mask painting area, which determines the area that is affected by the mask. The painted content of an element may be restricted to this area.

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.

margin-box

The painted content is restricted to (clipped to) the masking 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 values content-box, padding-box, border-box and margin-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-clip.

7.6 Positioning Area: the mask-origin property

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.

margin-box

The position is relative to the margin 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, border-box and margin-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: 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 image will be clipped.

7.7 Sizing Mask Images: the mask-size property

Specifies the size of the mask images.

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

7.8 Mask Shorthand: the mask property

<mask-layer> = <mask-reference> <source-type>? || <position> [ / <bg-size> ]? ||
    <repeat-style> || <geometry-box> || [ <geometry-box> | no-clip ]

If one <geometry-box> value is present then it 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-image, 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 image.

The mask shorthand also resets mask-box 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-box has also been reset to allow the new styles to take effect.

Note: A future level of this specification will allow different mask layers for various additional masking operations like composited mask layers masking and serial layer masking.

7.9 The Mask Image Rendering Model

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

7.9.1 Mask processing

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.

8 Box Masks

With mask-box 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 box image.

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

The mask 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-box. 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-box-slice: 25 fill;
    mask-box-repeat: stretch;
    mask-box-source: url(mask.png);
}

8.1 Mask Image Source: the mask-box-source property

Specifies an image to be used as mask 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 image but does not mask the element.

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

8.2 Mask Image Slicing: the mask-box-slice property

This property specifies inward offsets from the top, right, bottom, and left edges of the mask image, dividing it into nine regions: four corners, four edges and a middle. The middle image part is discarded (treated as fully transparent black) unless the fill-box keyword is present.

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

8.3 Masking Areas: the mask-box-width property

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

8.4 Edge Overhang: the mask-box-outset property

The values specify the amount by which the mask box 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-box-width, a <number> represents a multiple of the corresponding border-width. Negative values are not allowed for any of the mask-box-outset values.

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

8.5 Mask Image Tiling: the mask-box-repeat property

This property specifies how the images for the sides and the middle part of the mask 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 box image parts is given in the section Masking with the mask-box

8.6 Mask Box Image Shorthand: the mask-box property

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

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

8.7 Masking with the mask-box

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

The application of the mask-box-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-box-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

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-source-type property

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

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

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

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

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

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

10 Security

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 [HTML5] specification for all <mask-source>, <clip-source> and <image> values on the mask-image, mask-box-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

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.

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:

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

Appendix B: DOM interfaces

Interface SVGClipPathElement

The SVGClipPathElement interface corresponds to the clipPath element.

interface SVGClipPathElement : SVGElement {
  readonly attribute SVGAnimatedEnumeration clipPathUnits;
  readonly attribute SVGAnimatedTransformList transform;
};

SVGClipPathElement implements SVGUnitTypes;

Attributes:

clipPathUnits (readonly SVGAnimatedEnumeration)

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

transform (readonly SVGAnimatedTransformList)

Corresponds to presentation attribute transform on the given element.

Interface SVGMaskElement

The SVGMaskElement interface corresponds to the mask element.

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;
};

SVGMaskElement implements SVGUnitTypes;

Attributes:

maskUnits (readonly SVGAnimatedEnumeration)

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

maskContentUnits (readonly SVGAnimatedEnumeration)

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

x (readonly SVGAnimatedLength)

Corresponds to attribute x on the given mask element.

y (readonly SVGAnimatedLength)

Corresponds to attribute y on the given mask element.

width (readonly SVGAnimatedLength)

Corresponds to attribute width on the given mask element.

height (readonly SVGAnimatedLength)

Corresponds to attribute height on the given mask element.

Changes since last publication

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

• Remove note that a future version of the spec will allow controlling hit testing on clipping.
• Changed order of sections within the document.
• Make clear that the used value of the properties mask-image, mask-repeat, mask-position, mask-clip, mask-origin and mask-size must be ignored for <mask-source>, not the properties.
• "Animatable" section of mask-size, mask-position described as repeatable list of lists. Changed to a single list.
• Computed value of mask-repeat and mask-position was described as list of items. Changed to have just one value.
• Change link to ED from http://dev.w3.org/fxtf/masking/ to http://dev.w3.org/fxtf/css-masking-1/

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

• mask resets mask-box properties.
• Initial values for mask-repeat, mask-position and mask-origin changed to no-repeat, center and border-box.
• Multiple layers of mask images were deferred to a future level of this specification.
• Added security model for pixel operations and fetching of masking and clipping resources.
• Deferred child and select() function to next level.

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

• Better integration with terms and definitions of CSS Backgrounds and Borders module.
• Syntax changes on mask shorthand property to be conform with background shorthand property.
• Define how the implementation can differ between an SVG resource (mask, clipPath) and an image resource.
• Added mask-type property to alter between luminance and alpha mask on mask-image.
• Adapt IDL definition of SVGMaskElement and SVGClipPathElement to WebIDL.
• Further editorial changes.

See detailed list of changes in the ChangeLog.

Acknowledgments

Thanks to Elika J. Etemad, Liam R. E. Quin, Björn Höhrmann and Alan Stearns 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:

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 CSS2.1 specification reserves a prefixed syntax for proprietary and experimental extensions to CSS.

Prior to a specification reaching the Candidate Recommendation stage in the W3C process, all implementations of a CSS feature are considered experimental. The CSS Working Group recommends that implementations use a vendor-prefixed syntax for such features, including those in W3C Working Drafts. This avoids incompatibilities with future changes in the draft.

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.

References

Normative References

Informative References

Index

Property index