To allow various HTML embedded content elements to be used directly in SVG and support dynamic loading of content.
Owner:
Nobody
12.1. Overview
Embedded content is content that imports another resource into the document, or content from another vocabulary that is inserted into the document.
This is the same definition as HTML'sembedded content.
SVG supports embedded content with the use of ‘image’ and ‘foreignObject’ elements.
Except ‘canvas’ and ‘foreignObject’, embedded content is compatible with Resource Hints for prioritizing downloading of external resources.
12.2. Placement of the embedded content
The x, y, width, and height geometry properties specify the rectangular region into which the embedded content is positioned
(the positioning rectangle).
The positioning rectangle is used as the bounding box of the element;
note, however, that graphics may overflow the positioning rectangle,
depending on the value of the overflow property.
The elements in the HTML namespace do not have SVG presentation attributes
for the geometry properties.
Most of these elements, however, accept the HTML
width and heightdimensional attributes,
which are used as presentational hints
to set default values for the corresponding sizing properties.
The HTML dimensional attributes must be parsed and interpretted
as defined in the HTML specification [HTML].
Specifically, they only accept integer values, not CSS lengths with units.
On a ‘canvas’ element,
the attributes are slightly different:
they affect the rendered bitmap, not only its layout.
The x and y geometry properties
can only be set on HTML-namespaced elements via CSS.
When the embedded content consists of a document fragment
(e.g., a ‘foreignObject’,
an ‘audio’ or ‘video’ with user-agent generated controls,
or a ‘video’, ‘audio’, or ‘canvas’ displaying HTML fallback content),
the positioning rectangle defines the bounds of a
containing block for laying out the child content using CSS.
The scale of the containing block is defined in the current coordinate system,
including all explicit and implicit (e.g., ‘viewBox’) transformations.
The ‘foreignObject’, or other element that is positioned using SVG layout attributes,
is implicitly absolutely-positioned for the purposes of CSS layout.
As a result, any absolutely-positioned child elements
are positioned relative to this containing block.
Again, the overflow property determines
whether content that extends outside the positioning rectangle will be hidden.
A value of zero for either width or height disables rendering of the element and its embedded content.
The 'auto' value for width or height is used to size the corresponding element automatically based on the intrinsic dimensions or intrinsic aspect ratio of the referenced resource.
Computation of automatically-sized values follows the
Default Sizing Algorithm
defined for replaced elements in CSS layout [css-images-3].
In particular, when the referenced resource does not have an intrinsic size
(such as an ‘iframe’ or an image types with no defined dimensions),
it is assumed to have a width of 300px and a height of 150px.
CSS positioning properties (e.g. top and margin) have no effect when positioning the embedded content element in the SVG coordinate system.
They can, however, be used to position child elements of a ‘foreignObject’ or HTML embedding element.
The ‘image’ element
indicates that the contents of a complete file are to be
rendered into a given rectangle within the current user
coordinate system. The ‘image’ element can refer to raster
image files such as PNG or JPEG or to files with MIME type of
"image/svg+xml". Conforming SVG
viewers need to support at least PNG, JPEG and SVG format
files.
SVG files must be processed in secure animated mode
if the current document supports animation,
or in secure static mode if the current document is static.
The result of processing an ‘image’ is always a four-channel
RGBA result. When an ‘image’
element references an image (such as many PNG or JPEG
files) which only has three channels (RGB), then the effect is
as if the object were converted into a 4-channel RGBA image
with the alpha channel uniformly set to 1. For a single-channel (grayscale)
raster image, the effect is as if the object were converted
into a 4-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.
The ‘preserveAspectRatio’ calculations
are applied after determining the concrete object size,
and only have an effect if that size does not match the
intrinsic aspect ratio of the embedded image.
If a value of object-fit is used that
ensures that the concrete object size matches the intrinsic aspect ratio
(i.e., any value other than the default fill),
then the ‘preserveAspectRatio’ value will have no effect;
the image-rendering rectangle will be that determined
when scaling and positioning the object with CSS.
The ‘preserveAspectRatio’ attribute can therefore be safely used
as a fallback for most values of object-fit and object-position;
it must be explicitly set to none
to turn off aspect ratio control, regardless of object-fit value.
For example, if the image element referenced a PNG or JPEG
and preserveAspectRatio="xMinYMin
meet", then the aspect ratio of the raster would be
preserved (which means that the scale factor from image's
coordinates to current user space coordinates would be the same
for both X and Y), the raster would be sized as large as
possible while ensuring that the entire raster fits within the
viewport, and the top/left of the raster would be aligned with
the top/left of the viewport as defined by the attributes x, y, width and height on the ‘image’ element. If the value
of ‘preserveAspectRatio’ was 'none'
then aspect ratio of the image would not be preserved. The
image would be fit such that the top/left corner of the
raster exactly aligns with coordinate (x, y) and the bottom/right corner of
the raster exactly aligns with coordinate (x+width, y+height).
For ‘image’ elements embedding an SVG image,
the ‘preserveAspectRatio’ attribute on the root
element in the referenced SVG image must be ignored,
and instead treated as if it had a value of none.
(see ‘preserveAspectRatio’ for details).
This ensures that the ‘preserveAspectRatio’ attribute on
the referencing ‘image’ has its intended effect,
even if it is none.
When the value of the ‘preserveAspectRatio’ attribute on the ‘image’
is notnone,
the image-rendering rectangle determined
from the properties of the ‘image’ element
will exactly match the embedded SVG's intrinsic aspect ratio.
Ignoring the ‘preserveAspectRatio’ attribute from the embedded SVG
will therefore not usually have any effect.
The exception is if the aspect ratio of that image
is determined from absolute values for the width and height attributes
which do not match its ‘viewBox’ aspect ratio.
This is an unusual situation that authors are advised to avoid, for many reasons.
The user agent stylesheet sets the value of the overflow property
on ‘image’ element to hidden.
Unless over-ridden by the author, images will therefore be clipped to
the positioning rectangle defined by the geometry properties.
New in SVG 2.
Previous versions of SVG required that the overflow (and also clip)
property on the embedded SVG be ignored.
The new rules ensure that an overflowing slice layout
can be safely used without compromising the overflow control from the referenced image.
The resource referenced by the ‘image’ element represents a
separate document which generates its own parse tree and
document object model (if the resource is XML). Thus, there is
no inheritance of properties into the image.
Unlike ‘use’, the ‘image’ element cannot reference
elements within an SVG file.
Since image references always refer to a complete document,
a target-only URL is treated as a link to the same file,
which is rendered again as an independent embedded image.
Since the embedded image is processed in a secure mode,
its own embedded references are not processed,
preventing infinite recursion.
<?xml version="1.0" standalone="no"?>
<svg width="5cm" height="3cm" viewBox="0 0 50 30"
xmlns="http://www.w3.org/2000/svg"
xmlns:xlink="http://www.w3.org/1999/xlink">
<title>Recursive SVG</title>
<desc>An SVG with two recursive image reference to itself.
One reference uses the file name as a relative URL,
the other uses a target fragment only.
When viewed in a processing mode that supports external file references,
the embedded images should be rendered;
however, the embedded image must be processed in secure mode,
so the recursion only happens once.
The appearance should be three nested red circles in a bulls-eye pattern;
the innermost circle has solid fill because of target styles.
</desc>
<style type="text/css">
#target:target {
fill: red;
}
</style>
<circle id="target"
stroke="red" stroke-width="5" fill="none"
cx="50%" cy="50%" r="12" />
<image xlink:href="recursive-image.svg"
x="25%" y="25%" width="50%" height="50%" />
<image xlink:href="#target"
x="45%" y="45%" width="10%" height="10%" />
</svg>
HTML elements, in the HTML namespace, used as children of 'video', 'audio', 'iframe' and 'canvas' elements within an SVG document fragment behave as specified in HTML. This applies in particular to fallback content;
if fallback content is rendered,
the embedded element behaves like an SVG ‘foreignObject’ element to contain the HTML content.
This would occur, for example,
for a ‘video’ element if the user agent does not support the specified video formats,
or for a ‘canvas’ element if scripting is disabled.
<svg xmlns="http://www.w3.org/2000/svg" xmlns:html="http://www.w3.org/1999/xhtml">
<html:video src="http://example.org/dummyvideo" controls="controls">
<html:p>The video format is not supported by this browser.</html:p>
</html:video>
</svg>
The HTML specification is applicable also for the 'track' and 'source' elements.
Currently, within an SVG subtree, these tagnames are not recognized by the HTML parser
to be HTML-namespaced elements, although this may change in the future.
Therefore, in order to include these elements within SVG, one of the following
must be used:
An XML serialization that recognizes namespace declarations (such as stand-alone SVG or XHTML).
Namespaced elements as created via the createElementNS DOM API method.
A ‘foreignObject’ element to wrap the HTML-namespaced elements, which will then be correctly parsed.
Other HTML elements in an SVG subtree,
other than those inside a ‘foreignObject’ element,
must be treated as unknown elements for rendering purposes.
Many HTML elements will be treated as a parse error by the HTML parser,
causing the SVG fragment to terminate.
SVG is designed to be compatible with other XML languages
for describing and rendering other types of content.
The ‘foreignObject’
element allows for inclusion of elements in a non-SVG namespace which
is rendered within a region of the SVG graphic using other user agent processes. The
included foreign graphical content is subject to SVG
transformations, filters, clipping, masking and compositing.
Examples include
inserting a MathML expression into
an SVG drawing [MathML3],
or adding a block of complex CSS-formatted HTML text or form inputs.
The HTML parser treats elements inside the ‘foreignObject’
equivalent to elements inside an HTML document fragment.
Any svg or math element,
and their descendents,
will be parsed as being in the SVG or MathML namespace, respectively;
all other tags will be parsed as being in the HTML namespace.
SVG-namespaced elements within a ‘foreignObject’ will not be rendered,
except in the situation where a properly defined SVG
fragment, including a root ‘svg’ element is
defined as a descendent of the ‘foreignObject’.
A ‘foreignObject’
may be used in conjunction with the ‘switch’ element and
the ‘requiredExtensions’ attribute to
provide proper checking for user agent support and provide an
alternate rendering in case user agent support is not
available.
This specification does not define how ‘requiredExtensions’
values should be mapped to support for different XML languages;
a future specification may do so.
It is not required that SVG user agent support the ability
to invoke other arbitrary user agents to handle embedded
foreign object types; however, all conforming SVG user agents
would need to support the ‘switch’ element and
must be able to render valid SVG elements when they appear as
one of the alternatives within a ‘switch’
element.
It is expected that commercial Web browsers will
support the ability for SVG to embed
CSS-formatted HTML and also MathML content,
with the rendered content subject to transformations and compositing defined in the SVG fragment.
At this time, such a capability is not a requirement.
<?xml version="1.0" standalone="yes"?>
<svg width="4in" height="3in"
xmlns = 'http://www.w3.org/2000/svg'>
<desc>This example uses the 'switch' element to provide a
fallback graphical representation of an paragraph, if
XMHTML is not supported.</desc>
<!-- The 'switch' element will process the first child element
whose testing attributes evaluate to true.-->
<switch>
<!-- Process the embedded XHTML if the requiredExtensions attribute
evaluates to true (i.e., the user agent supports XHTML
embedded within SVG). -->
<foreignObject width="100" height="50"
requiredExtensions="http://example.com/SVGExtensions/EmbeddedXHTML">
<!-- XHTML content goes here -->
<body xmlns="http://www.w3.org/1999/xhtml">
<p>Here is a paragraph that requires word wrap</p>
</body>
</foreignObject>
<!-- Else, process the following alternate SVG.
Note that there are no testing attributes on the 'text' element.
If no testing attributes are provided, it is as if there
were testing attributes and they evaluated to true.-->
<text font-size="10" font-family="Verdana">
<tspan x="10" y="10">Here is a paragraph that</tspan>
<tspan x="10" y="20">requires word wrap.</tspan>
</text>
</switch>
</svg>
The
x,
y,
width and
height IDL attributes
reflect the computed values of the x, y, width and
height properties and their corresponding
presentation attributes, respectively.
The
x,
y,
width and
height IDL attributes
reflect the computed values of the x, y, width and
height properties and their corresponding
presentation attributes, respectively.
