first

Daisy Diff compare report. Click on the changed parts for a detailed description. Use the left and right arrow keys to walk through the modifications.

last

5 Document Structure

Contents

• 5.1 Defining an SVG document fragment: the 'svg' element
  • 5.1.1 Overview
  • 5.1.2 The 'svg' element
• 5.2 Grouping: the 'g' element
  • 5.2.1 Overview
  • 5.2.2 The 'g' element
• 5.3 References and the 'defs' element
  • 5.3.1 Overview
  • 5.3.2 URI reference attributes
  • 5.3.3 The 'defs' element
• 5.4 The 'desc' and 'title' elements
• 5.5 The 'symbol' element
• 5.6 The 'use' element
• 5.7 The 'image' element
• 5.8 Conditional processing
  • 5.8.1 Conditional processing overview
  • 5.8.2 The 'switch' element
  • 5.8.3 The 'requiredFeatures' attribute
  • 5.8.4 The 'requiredExtensions' attribute
  • 5.8.5 The 'systemLanguage' attribute
  • 5.8.6 Applicability of test attributes
• 5.9 Specifying whether external resources are required for proper rendering
• 5.10 Common attributes
  • 5.10.1 Attributes common to all elements: 'id' and 'xml:base'
  • 5.10.2 The 'xml:lang' and 'xml:space' attributes
• 5.11
Core Attribute Module
• DOM interfaces
  • 5.
12 Structure Module
• 5.13 Basic Structure Module
• 5.14 Container Attribute Module
• 5.15 Conditional Processing Module
• 5.16 Image Module
• 5.17 DOM interfaces

• • 11.1 Interface SVGDocument
  • 5.11.2 Interface SVGSVGElement
  • 5.11.3 Interface SVGGElement
  • 5.11.4 Interface SVGDefsElement
  • 5.11.5 Interface SVGDescElement
  • 5.11.6 Interface SVGTitleElement
  • 5.11.7 Interface SVGSymbolElement
  • 5.11.8 Interface SVGUseElement
  • 5.11.9 Interface SVGElementInstance
  • 5.11.10 Interface SVGElementInstanceList
  • 5.11.11 Interface SVGImageElement
  • 5.11.12 Interface SVGSwitchElement
  • 5.11.13 Interface GetSVGDocument

5.1 Defining an SVG document fragment: the 'svg' element

5.1.1 Overview

An SVG document fragment consists of any number of SVG elements contained within an 'svg' element.

An SVG document fragment can range from an empty fragment (i.e., no content inside of the 'svg' element), to a very simple SVG document fragment containing a single SVG graphics element such as a 'rect', to a complex, deeply nested collection of container elements and graphics elements.

An SVG document fragment can stand by itself as a self-contained file or resource, in which case the SVG document fragment is an SVG document, or it can be embedded inline as a fragment within a parent XML document.

The following example shows simple SVG content embedded inline as a fragment within a parent XML document. Note the use of XML namespaces to indicate that the 'svg' and 'ellipse' elements belong to the SVG namespace:

<?xml version="1.0" standalone="yes"?>
<parent xmlns="http://example.org"
        xmlns:svg="http://www.w3.org/2000/svg">
   <!-- parent contents here -->
   <svg:svg width="4cm" height="8cm" version="1.1">
      <svg:ellipse cx="2cm" cy="4cm" rx="2cm" ry="1cm" />
   </svg:svg>
   <!-- ... -->
</parent>

This example shows a slightly more complex (i.e., it contains multiple rectangles) stand-alone, self-contained SVG document:

<?xml version="1.0" standalone="no"?>
<!DOCTYPE svg PUBLIC "-//W3C//DTD SVG 1.1//EN" 
  "http://www.w3.org/Graphics/SVG/1.1/DTD/svg11.dtd">
<svg width="5cm" height="4cm" version="1.1"
     xmlns="http://www.w3.org/2000/svg">
  <desc>Four separate rectangles
  </desc>
    <rect x="0.5cm" y="0.5cm" width="2cm" height="1cm"/>
    <rect x="0.5cm" y="2cm" width="1cm" height="1.5cm"/>
    <rect x="3cm" y="0.5cm" width="1.5cm" height="2cm"/>
    <rect x="3.5cm" y="3cm" width="1cm" height="0.5cm"/>

  <!-- Show outline of canvas using 'rect' element -->
  <rect x=".01cm" y=".01cm" width="4.98cm" height="3.98cm"
        fill="none" stroke="blue" stroke-width=".02cm" />

</svg>

'svg' elements can appear in the middle of SVG content. This is the mechanism by which SVG document fragments can be embedded within other SVG document fragments.

Another use for 'svg' elements within the middle of SVG content is to establish a new viewport. (See Establishing a new viewport.)

In all cases, for compliance with the "Namespaces in XML" Recommendation [XML-NS], an SVG namespace declaration must be provided so that all SVG elements are identified as belonging to the SVG namespace. The following are possible ways to provide a namespace declaration. An 'xmlns' attribute without a namespace prefix could be specified on an 'svg' element, which means that SVG is the default namespace for all elements within the scope of the element with the 'xmlns' attribute:

<svg xmlns="http://www.w3.org/2000/svg"

 …>
  <rect 

…/>
</svg>

If a namespace prefix is specified on the 'xmlns' attribute (e.g., xmlns:svg="http://www.w3.org/2000/svg"), then the corresponding namespace is not the default namespace, so an explicit namespace prefix must be assigned to the elements:

<svg:svg xmlns:svg="http://www.w3.org/2000/svg"

 …>
  <svg:rect 

…/>
</svg:svg>

Namespace prefixes can be specified on ancestor elements (illustrated in the above example). For more information, refer to the "Namespaces in XML" Recommendation [XML-NS].

5.1.2 The 'svg' element

<!-- svg: SVG Document Element ......................... -->
<!ENTITY % SVG.svg.extra.content "" >
<!ENTITY % SVG.svg.element "INCLUDE" >
<![%SVG.svg.element;[
<!ENTITY % SVG.svg.content
    "( %SVG.Description.class; | %SVG.Animation.class; %SVG.Structure.class;
       %SVG.Conditional.class; %SVG.Image.class; %SVG.Style.class;
       %SVG.Shape.class; %SVG.Text.class; %SVG.Marker.class;
       %SVG.ColorProfile.class; %SVG.Gradient.class; %SVG.Pattern.class;
       %SVG.Clip.class; %SVG.Mask.class; %SVG.Filter.class; %SVG.Cursor.class;
       %SVG.Hyperlink.class; %SVG.View.class; %SVG.Script.class;
       %SVG.Font.class; %SVG.svg.extra.content; )*"
>
<!ELEMENT %SVG.svg.qname; %SVG.svg.content; >
<!-- end of SVG.svg.element -->]]>
<!ENTITY % SVG.svg.attlist "INCLUDE" >
<![%SVG.svg.attlist;[
<!ATTLIST %SVG.svg.qname;
    %SVG.xmlns.attrib;
    %SVG.Core.attrib;
    %SVG.Conditional.attrib;
    %SVG.Style.attrib;
    %SVG.Presentation.attrib;
    %SVG.DocumentEvents.attrib;
    %SVG.GraphicalEvents.attrib;
    %SVG.External.attrib;
    x %Coordinate.datatype; #IMPLIED
    y %Coordinate.datatype; #IMPLIED
    width %Length.datatype; #IMPLIED
    height %Length.datatype; #IMPLIED
    viewBox %ViewBoxSpec.datatype; #IMPLIED
    preserveAspectRatio %PreserveAspectRatioSpec.datatype; 'xMidYMid meet'
    zoomAndPan ( disable | magnify ) 'magnify'
    version %Number.datatype; #FIXED '1.1'
    baseProfile %Text.datatype; #IMPLIED
    contentScriptType %ContentType.datatype; 'text/ecmascript'
    contentStyleType %ContentType.datatype; 'text/css'
>
<!-- end of SVG.svg.attlist -->]]>

Attribute definitions:

If an SVG document is likely to be referenced as a component of another document, the author will often want to include a 'viewBox' attribute on the outermost 'svg' element of the referenced document. This attribute provides a convenient way to design SVG documents to scale-to-fit into an arbitrary viewport.

5.2 Grouping: the 'g' element

5.2.1 Overview

The 'g' element is a container element for grouping together related graphics elements.

Grouping constructs, when used in conjunction with the 'desc' and 'title' elements, provide information about document structure and semantics. Documents that are rich in structure may be rendered graphically, as speech, or as braille, and thus promote accessibility.

A group of elements, as well as individual objects, can be given a name using the 'id' attribute. Named groups are needed for several purposes such as animation and re-usable objects.

An example:

<?xml version="1.0" standalone="no"?>
<!DOCTYPE svg PUBLIC "-//W3C//DTD SVG 1.1//EN" 
  "http://www.w3.org/Graphics/SVG/1.1/DTD/svg11.dtd">
<svg 

xmlns="http://www.w3.org/2000/svg"
     version="1.1" width="5cm" height="5cm">
  <desc>Two groups, each of two 

rectangles</desc>
  <g id="group1" fill="red"

>
    <rect x="1cm" y="1cm" width="1cm" height="1cm"

/>
    <rect x="3cm" y="1cm" width="1cm" height="1cm"

/>
  </g>
  <g id="group2" fill="blue"

>
    <rect x="1cm" y="3cm" width="1cm" height="1cm"

/>
    <rect x="3cm" y="3cm" width="1cm" height="1cm"

/>
  </g>

  <!-- Show outline of canvas using 'rect' element -->
  <rect x=".01cm" y=".01cm" width="4.98cm" height="4.98cm"
        fill="none" stroke="blue" stroke-width=".02cm"

/>
</svg>

View this example as SVG (SVG-enabled browsers only)  

A 'g' element can contain other 'g' elements nested within it, to an arbitrary depth. Thus, the following is possible:

<?xml version="1.0" standalone="no"?>
<!DOCTYPE svg PUBLIC "-//W3C//DTD SVG 1.1//EN" 
  "http://www.w3.org/Graphics/SVG/1.1/DTD/svg11.dtd">
<svg 

xmlns="http://www.w3.org/2000/svg"
     version="1.1" width="4in" height="3in">
  <desc>Groups can 

nest</desc>
  <g>
     <g>
       <g>
       </g>
     </g>
   </g>
</svg>

Any element that is not contained within a 'g' is treated (at least conceptually) as if it were in its own group.

5.2.2 The 'g' element

<!ENTITY % SVG.g.extra.content "" >
<!ENTITY % SVG.g.element "INCLUDE" >
<![%SVG.g.element;[
<!ENTITY % SVG.g.content
    "( %SVG.Description.class; | %SVG.Animation.class; %SVG.Structure.class;
       %SVG.Conditional.class; %SVG.Image.class; %SVG.Style.class;
       %SVG.Shape.class; %SVG.Text.class; %SVG.Marker.class;
       %SVG.ColorProfile.class; %SVG.Gradient.class; %SVG.Pattern.class;
       %SVG.Clip.class; %SVG.Mask.class; %SVG.Filter.class; %SVG.Cursor.class;
       %SVG.Hyperlink.class; %SVG.View.class; %SVG.Script.class;
       %SVG.Font.class; %SVG.g.extra.content; )*"
>
<!ELEMENT %SVG.g.qname; %SVG.g.content; >
<!-- end of SVG.g.element -->]]>
<!ENTITY % SVG.g.attlist "INCLUDE" >

5.3 References and the 'defs' element

5.3.1 Overview

SVG makes extensive use of URI references [URIRFC2396] to other objects. For example, to fill a rectangle with a linear gradient, you first define a 'linearGradient' element and give it an ID, as in:

<linearGradient id="MyGradient">...</linearGradient>

You then reference the linear gradient as the value of the 'fill' property for the rectangle, as in:

<rect style="fill:url(#MyGradient)"/>

URI references are defined in either of the following forms:

<URI-reference> = [ <absoluteURI> | <relativeURI> ] [ "#" <elementID> ]    -or-
<URI-reference> = [ <absoluteURI> | <relativeURI> ] [ "#xpointer(id(" <elementID> "))" ]

where <elementID> is the ID of the referenced element.

(Note that the two forms above (i.e., #<elementID> and #xpointer(id(<elementID>))) are formulated in syntaxes compatible with "XML Pointer Language (XPointer)" [XPTR]. These two formulations of URI references are the only XPointer formulations that are required in SVG 1.0 1 user agents.)

SVG supports two types of URI references:

• local URI references, where the URI reference does not contain an <absoluteURI> or <relativeURI> and thus only contains a fragment identifier (i.e., #<elementID> or #xpointer(id<elementID>))
• non-local URI references, where the URI reference does contain an <absoluteURI> or <relativeURI>

The following rules apply to the processing of URI references:

• URI references to nodes that do not exist shall be treated as invalid references.
• URI references to elements which are inappropriate targets for the given reference shall be treated as invalid references (see list below for appropriate targets). For example, the 'clip-path' property can only refer to 'clipPath' elements. The property setting clip-path:url(#MyElement) is an invalid reference if the referenced element is not a 'clipPath'.
• URI references that directly or indirectly reference themselves are treated as invalid circular references.

The following list describes the elements and properties that allow URI references and the valid target types for those references:

• the 'a' element can reference any local or non-local resource
• the 'altGlyph' element must reference either an 'altGlyphDef' element or a 'glyph' element
• the 'animate' element (see Identifying the target element for an animation for reference rules)
• the 'animateColor' element (see Identifying the target element for an animation for reference rules)
• the 'animateMotion' element (see Identifying the target element for an animation for reference rules)
• the 'animateTransform' element (see Identifying the target element for an animation for reference rules)
• the 'clip-path' property must reference a 'clipPath' element
• the 'color-profile' element must reference an ICC profile resource
• the 'color-profile' property must reference an ICC profile resource or a 'color-profile' element
• the 'src' descriptor on an @color-profile definition must reference an ICC profile resource or a 'color-profile' element
• the 'cursor' element must reference a resource that can provide an image for the cursor graphic
• the 'cursor' property must reference a resource that can provide an image for the cursor graphic
• the 'feImage' element must reference any local or non-local resource
• the 'fill' property (see Specifying paint for reference rules)
• the 'filter' element must reference a 'filter' element
• the 'filter' property must reference a 'filter' element
• the 'image' element must reference any local or non-local resource
• the 'linearGradient' element must reference a 'linearGradient' or 'radialGradient' element
• the 'marker', 'marker-start', 'marker-mid' and 'marker-end' properties must reference a 'marker' element.
• the 'mask' property must reference a 'mask' element
• the 'pattern' element must reference a 'pattern' element
• the 'radialGradient' element must reference a 'linearGradient' or 'radialGradient' element
• the 'script' element must reference an external resource that provides the script content
• the 'stroke' property (see Specifying paint for reference rules)
• the 'textpathtextPath' element must reference a 'path' element
• the 'tref' element can reference any SVG element
• the 'set' element (see Identifying the target element for an animation for reference rules)
• the 'use' element can reference any local or non-local resource

The following rules apply to the processing of invalid URI references:

• An invalid local URI reference (i.e., an invalid references to a node within the current document) represents an error (see Error processing), apart from the 'xlink:href' attribute on the 'a' element and the properties that allow for backup values in the case where the URI reference is invalid (see 'fill' and 'stroke').
• An invalid circular URI reference represents an error (see Error processing).
• When attribute 'externalResourcesRequired' has been set to 'true' on the referencing element or one of its ancestors, then an unresolved external URI reference (i.e., a resource that cannot be located) represents an error (see Error processing).

It is recommended that, wherever possible, referenced elements be defined inside of a 'defs' element. Among the elements that are always referenced: 'altGlyphDef', 'clipPath', 'cursor', 'filter', 'linearGradient', 'marker', 'mask', 'pattern', 'radialGradient' and 'symbol'. Defining these elements inside of a 'defs' element promotes understandability of the SVG content and thus promotes accessibility.

5.3.2 URI reference attributes

A URI reference is specified within an 'href' attribute in the XLink [XLINK] namespace. If the default common prefix of '"xlink:' " is used for attributes in the XLink namespace, then the attribute will be specified as 'xlink:href'. The value of this attribute is a URI reference for the desired resource (or resource fragment).

The value of the 'href' attribute must be a URI reference as defined in [RFC2396], or must result in a URI reference after the escaping procedure described below is applied. The procedure is applied when passing the URI reference to a URI resolver.

Some characters are disallowed in URI references, even if they are allowed in XML; the disallowed characters include all non-ASCII characters, plus the excluded characters listed in Section 2.4 of Uniform Resource Identifiers (URI): Generic Syntax [RFC2396], except for the number sign (#) and percent sign (%) and the square bracket characters re-allowed in Format for Literal IPv6 Addresses in URLs [RFC2732]. Disallowed characters must be escaped as follows:

1. Each disallowed character is converted to UTF-8 [RFC2279RFC3629] as one or more bytes.

2. Any bytes corresponding to a disallowed character are escaped with the URI escaping mechanism (that is, converted to %HH, where HH is the hexadecimal notation of the byte value).

3. The original character is replaced by the resulting character sequence.

Because it is impractical for any application to check that a value is a URI reference, this specification follows the lead of Uniform Resource Identifiers (URI): Generic Syntax [RFC2396] in this matter and imposes no such conformance testing requirement on SVG applications.

If the URI reference is relative, its absolute version must be computed by the method of specified in XML Base [XML-BaseBASE] before use.

For locators into XML resources, the format of the fragment identifier (if any) used within the URI reference is specified by the XPointer specification [XPTR].

Additional XLink attributes can be specified that provide supplemental information regarding the referenced resource. These additional attributes are included in the DTD in the following entities. The three entity definitions differ only in the value of xlink:show, which has the value other in the first two entities and the value embed in the third. The first two entity definitions are used in most element definitions which reference resources. The third entity definition is used by elements 'use', 'image' and 'feImage'.

<!ENTITY % SVG.XLink.extra.attrib "" >
<!ENTITY % SVG.XLink.attrib
    "%XLINK.xmlns.attrib;
     %XLINK.pfx;type ( simple ) #FIXED 'simple'
     %XLINK.pfx;href %URI.datatype; #IMPLIED
     %XLINK.pfx;role %URI.datatype; #IMPLIED
     %XLINK.pfx;arcrole %URI.datatype; #IMPLIED
     %XLINK.pfx;title CDATA #IMPLIED
     %XLINK.pfx;show ( other ) 'other'
     %XLINK.pfx;actuate ( onLoad ) #FIXED 'onLoad'
     %SVG.XLink.extra.attrib;"
>
<!ENTITY % SVG.XLinkRequired.extra.attrib "" >
<!ENTITY % SVG.XLinkRequired.attrib
    "%XLINK.xmlns.attrib;
     %XLINK.pfx;type ( simple ) #FIXED 'simple'
     %XLINK.pfx;href %URI.datatype; #REQUIRED
     %XLINK.pfx;role %URI.datatype; #IMPLIED
     %XLINK.pfx;arcrole %URI.datatype; #IMPLIED
     %XLINK.pfx;title CDATA #IMPLIED
     %XLINK.pfx;show ( other ) 'other'
     %XLINK.pfx;actuate ( onLoad ) #FIXED 'onLoad'
     %SVG.XLinkRequired.extra.attrib;"
>
<!ENTITY % SVG.XLinkEmbed.extra.attrib "" >
<!ENTITY % SVG.XLinkEmbed.attrib
    "%XLINK.xmlns.attrib;
     %XLINK.pfx;type ( simple ) #FIXED 'simple'
     %XLINK.pfx;href %URI.datatype; #REQUIRED
     %XLINK.pfx;role %URI.datatype; #IMPLIED
     %XLINK.pfx;arcrole %URI.datatype; #IMPLIED
     %XLINK.pfx;title CDATA #IMPLIED
     %XLINK.pfx;show ( embed ) 'embed'
     %XLINK.pfx;actuate ( onLoad ) #FIXED 'onLoad'
     %SVG.XLinkEmbed.extra.attrib;"
>
<!ENTITY % SVG.XLinkReplace.extra.attrib "" >
<!ENTITY % SVG.XLinkReplace.attrib
    "%XLINK.xmlns.attrib;
     %XLINK.pfx;type ( simple ) #FIXED 'simple'
     %XLINK.pfx;href %URI.datatype; #REQUIRED
     %XLINK.pfx;role %URI.datatype; #IMPLIED
     %XLINK.pfx;arcrole %URI.datatype; #IMPLIED
     %XLINK.pfx;title CDATA #IMPLIED
     %XLINK.pfx;show ( new | replace ) 'replace'
     %XLINK.pfx;actuate ( onRequest ) #FIXED 'onRequest'
     %SVG.XLinkReplace.extra.attrib;"
>

In all cases, for compliance with the "Namespaces in XML" Recommendation [XML-NS], an explicit XLink namespace declaration must be provided whenever one of the above XLink attributes is used within SVG content. One simple way to provide such an XLink namespace declaration is to include an 'xmlns' attribute for the XLink namespace on the outermost 'svg' element for content that uses XLink attributes. For example:

<svg xmlns:xlink="http://www.w3.org/1999/xlink" ...>
  <image xlink:href="foo.png" .../>
</svg>

5.3.3 The 'defs' element

The 'defs' element is a container element for referenced elements. For understandability and accessibility reasons, it is recommended that, whenever possible, referenced elements be defined inside of a 'defs'.

The content model for 'defs' is the same as for the 'g' element; thus, any element that can be a child of a 'g' can also be a child of a 'defs', and vice versa.

Elements that are descendants of a 'defs' are not rendered directly; they are prevented from becoming part of the rendering tree just as if the 'defs' element were a 'g' element and the 'display' property were set to none. Note, however, that the descendants of a 'defs' are always present in the source tree and thus can always be referenced by other elements; thus, the value of the 'display' property on the 'defs' element or any of its descendants does not prevent those elements from being referenced by other elements.

<!ENTITY % SVG.defs.extra.content "" >
<!ENTITY % SVG.defs.element "INCLUDE" >
<![%SVG.defs.element;[
<!ENTITY % SVG.defs.content
    "( %SVG.Description.class; | %SVG.Animation.class; %SVG.Structure.class;
       %SVG.Conditional.class; %SVG.Image.class; %SVG.Style.class;
       %SVG.Shape.class; %SVG.Text.class; %SVG.Marker.class;
       %SVG.ColorProfile.class; %SVG.Gradient.class; %SVG.Pattern.class;
       %SVG.Clip.class; %SVG.Mask.class; %SVG.Filter.class; %SVG.Cursor.class;
       %SVG.Hyperlink.class; %SVG.View.class; %SVG.Script.class;
       %SVG.Font.class; %SVG.defs.extra.content; )*"
>
<!ELEMENT %SVG.defs.qname; %SVG.defs.content; >
<!-- end of SVG.defs.element -->]]>
<!ENTITY % SVG.defs.attlist "INCLUDE" >
<![%SVG.defs.attlist;[
<!ATTLIST %SVG.defs.qname;
    %SVG.Core.attrib;
    %SVG.Conditional.attrib;
    %SVG.Style.attrib;
    %SVG.Presentation.attrib;
    %SVG.GraphicalEvents.attrib;
    %SVG.External.attrib;
    transform %TransformList.datatype; #IMPLIED
>

To provide some SVG user agents with an opportunity to implement efficient implementations in streaming environments, creators of SVG content are encouraged to place all elements which are targets of local URI references within a 'defs' element which is a direct child of one of the ancestors of the referencing element. For example:

<?xml version="1.0" standalone="no"?>
<!DOCTYPE svg PUBLIC "-//W3C//DTD SVG 1.1//EN" "http://www.w3.org/Graphics/SVG/1.1/DTD/svg11.dtd">
<svg width="8cm" height="3cm"
     xmlns="http://www.w3.org/2000/svg" version="1.1">
  <desc>Local URI references within ancestor's 'defs' element.</desc>
  <defs>
    <linearGradient id="Gradient01">
      <stop offset="20%" stop-color="#39F" />
      <stop offset="90%" stop-color="#F3F" />
    </linearGradient>
  </defs>
  <rect x="1cm" y="1cm" width="6cm" height="1cm" 
        fill="url(#Gradient01)"  />

  <!-- Show outline of canvas using 'rect' element -->
  <rect x=".01cm" y=".01cm" width="7.98cm" height="2.98cm"
        fill="none" stroke="blue" stroke-width=".02cm" />

</svg>

View this example as SVG (SVG-enabled browsers only)  

In the document above, the linear gradient is defined within a 'defs' element which is the direct child of the 'svg' element, which in turn is an ancestor of the 'rect' element which references the linear gradient. Thus, the above document conforms to the guideline.

5.4 The 'desc' and 'title' elements

Each container element or graphics element in an SVG drawing can supply a 'desc' and/or a 'title' description string where the description is text-only. When the current SVG document fragment is rendered as SVG on visual media, 'desc' and 'title' elements are not rendered as part of the graphics. User agents may, however, for example, display the 'title' element as a tooltip, as the pointing device moves over particular elements. Alternate presentations are possible, both visual and aural, which display the 'desc' and 'title' elements but do not display 'path' elements or other graphics elements. This is readily achieved by using a different (perhaps user) style sheet. For deep hierarchies, and for following 'use' element references, it is sometimes desirable to allow the user to control how deep they drill down into descriptive text.

<!ENTITY % SVG.desc.extra.content "" >
<!ENTITY % SVG.desc.element "INCLUDE" >
<![%SVG.desc.element;[
<!ENTITY % SVG.desc.content
    "( #PCDATA %SVG.desc.extra.content; )*"
>
<!ELEMENT %SVG.desc.qname; %SVG.desc.content; >
<!-- end of SVG.desc.element -->]]>
<!ENTITY % SVG.desc.attlist "INCLUDE" >
<![%SVG.desc.attlist;[
<!ATTLIST %SVG.desc.qname;
    %SVG.Core.attrib;
    %SVG.Style.attrib;
>

<!ENTITY % SVG.title.extra.content "" >
<!ENTITY % SVG.title.element "INCLUDE" >
<![%SVG.title.element;[
<!ENTITY % SVG.title.content
    "( #PCDATA %SVG.title.extra.content; )*"
>
<!ELEMENT %SVG.title.qname; %SVG.title.content; >

The following is an example. In typical operation, the SVG user agent would not render the 'desc' and 'title' elements but would render the remaining contents of the 'g' element.

<?xml version="1.0" standalone="no"?>
<!DOCTYPE svg SYSTEM "http://www.w3.org/Graphics/SVG/1.1/DTD/svg11.dtd">
<svg width="4in" height="3in" version="1.1"
     xmlns="http://www.w3.org/2000/svg">
<g>   <title>  version="1.1" width="4in" height="3in">
Company sales by<g>
region   < <title>Company sales by region</title>
    <desc>
      This is a bar chart which shows 
      company sales by region.
    </desc>
    <!-- Bar chart defined as vector data -->
  </g>
</svg>

Description and title elements can contain marked-up text from other namespaces. Here is an example:

<?xml version="1.0" standalone="yes"?>
<svg width="4in" height="3in" version="1.1"
 xmlns="http://www.w3.org/2000/svg"
     version="1.1" width="4in" height="3in">
   <desc xmlns:mydoc="http://example.org/mydoc">
      <mydoc:title>This is an example SVG file</mydoc:title>
      <mydoc:para>The global description uses markup from the 
        <mydoc:emph>mydoc</mydoc:emph> namespace.</mydoc:para>
   </desc>
   <g>
    <!-- the picture goes here -->
   </g>
</svg>

Authors should always provide a 'title' child element to the outermost 'svg' element within a stand-alone SVG document. The 'title' child element to an 'svg' element serves the purposes of identifying the content of the given SVG document fragment. Since users often consult documents out of context, authors should provide context-rich titles. Thus, instead of a title such as "Introduction", which doesn’t doesn't provide much contextual background, authors should supply a title such as "Introduction to Medieval Bee-Keeping" instead. For reasons of accessibility, user agents should always make the content of the 'title' child element to the outermost 'svg' element available to users. The mechanism for doing so depends on the user agent (e.g., as a caption, spoken).

The DTD definitions of many of SVG's elements (particularly, container and text elements) place no restriction on the placement or number of the 'desc' and 'title' sub-elements. This flexibility is only present so that there will be a consistent content model for container elements, because some container elements in SVG allow for mixed content, and because the mixed content rules for XML [XML-MIXED] do not permit the desired restrictions. Representations of future versions of the SVG language might use more expressive representations than DTDs which allow for more restrictive mixed content rules. It is strongly recommended that at most one 'desc' and at most one 'title' element appear as a child of any particular element, and that these elements appear before any other child elements (except possibly 'metadata' elements) or character data content. If user agents need to choose among multiple 'desc' or 'title' elements for processing (e.g., to decide which string to use for a tooltip), the user agent shall choose the first one.

5.5 The 'symbol' element

The 'symbol' element is used to define graphical template objects which can be instantiated by a 'use' element.

The use of 'symbol' elements for graphics that are used multiple times in the same document adds structure and semantics. Documents that are rich in structure may be rendered graphically, as speech, or as braille, and thus promote accessibility.

The key distinctions between a 'symbol' and a 'g' are:

• A 'symbol' element itself is not rendered. Only instances of a 'symbol' element (i.e., a reference to a 'symbol' by a 'use' element) are rendered.
• A 'symbol' element has attributes 'viewBox' and 'preserveAspectRatio' which allow a 'symbol' to scale-to-fit within a rectangular viewport defined by the referencing 'use' element.

Closely related to the 'symbol' element are the 'marker' and 'pattern' elements.  

<!ENTITY % SVG.symbol.extra.content "" >
<!ENTITY % SVG.symbol.element "INCLUDE" >
<![%SVG.symbol.element;[
<!ENTITY % SVG.symbol.content
    "( %SVG.Description.class; | %SVG.Animation.class; %SVG.Structure.class;
       %SVG.Conditional.class; %SVG.Image.class; %SVG.Style.class;
       %SVG.Shape.class; %SVG.Text.class; %SVG.Marker.class;
       %SVG.ColorProfile.class; %SVG.Gradient.class; %SVG.Pattern.class;
       %SVG.Clip.class; %SVG.Mask.class; %SVG.Filter.class; %SVG.Cursor.class;
       %SVG.Hyperlink.class; %SVG.View.class; %SVG.Script.class;
       %SVG.Font.class; %SVG.symbol.extra.content; )*"
>
<!ELEMENT %SVG.symbol.qname; %SVG.symbol.content; >

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

5.6 The 'use' element

Any 'svg', 'symbol', 'g', graphics element or other 'use' is potentially a template object that can be re-used (i.e., "instanced") in the SVG document via a 'use' element. The 'use' element references another element and indicates that the graphical contents of that element is included/drawn at that given point in the document.

Unlike 'image', the 'use' element cannot reference entire files.

The 'use' element has optional attributes 'x', 'y', 'width' and 'height' which are used to map the graphical contents of the referenced element onto a rectangular region within the current coordinate system.

The effect of a 'use' element is as if the contents of the referenced element were deeply cloned into a separate non-exposed DOM tree which had the 'use' element as its parent and all of the 'use' element's ancestors as its higher-level ancestors. Because the cloned DOM tree is non-exposed, the SVG Document Object Model (DOM) only contains the 'use' element and its attributes. The SVG DOM does not show the referenced element's contents as children of 'use' element.

For user agents that support Styling with CSS, the conceptual deep cloning of the referenced element into a non-exposed DOM tree also copies any property values resulting from the CSS cascade ([CSS2-CASCADE], chapter 6) on the referenced element and its contents. CSS2 CSS 2 selectors can be applied to the original (i.e., referenced) elements because they are part of the formal document structure. CSS2 CSS 2 selectors cannot be applied to the (conceptually) cloned DOM tree because its contents are not part of the formal document structure.

Property inheritance, however, works as if the referenced element had been textually included as a deeply cloned child of the 'use' element. The referenced element inherits properties from the 'use' element and the 'use' element's ancestors. An instance of a referenced element does not inherit properties from the referenced element's original parents.

If event attributes are assigned to referenced elements, then the actual target for the event will be the SVGElementInstance object within the "instance tree" corresponding to the given referenced element.

The event handling for the non-exposed tree works as if the referenced element had been textually included as a deeply cloned child of the 'use' element, except that events are dispatched to the SVGElementInstance objects. The event's target and currentTarget attributes are set to the SVGElementInstance that corresponds to the target and current target elements in the referenced subtree. An event propagates through the exposed and non-exposed portions of the tree in the same manner as it would in the regular document tree: first going from the root element to the 'use' element and then through non-exposed tree elements in the capture phase, followed by the target phase at the target of the event, then bubbling back through non-exposed tree to the use element and then back through regular tree to the root element in bubbling phase.

An element and all its corresponding SVGElementInstance objects share an event listener list. The currentTarget attribute of the event can be used to determine through which object an event listener was invoked.

The behavior of the 'visibility' property conforms to this model of property inheritance. Thus, specifying 'visibility:hidden' on a 'use' element does not guarantee that the referenced content will not be rendered. If the 'use' element specifies 'visibility:hidden' and the element it references specifies 'visibility:hidden' or 'visibility:inherit', then that one element will be hidden. However, if the referenced element instead specifies 'visibility:visible', then that element will be visible even if the 'use' element specifies 'visibility:hidden'.

Animations on a referenced element will cause the instances to also be animated.

A 'use' element has the same visual effect as if the 'use' element were replaced by the following generated content:

• If the 'use' element references a 'symbol' element:
  
  In the generated content, the 'use' will be replaced by 'g', where all attributes from the 'use' element except for 'x', 'y', 'width', 'height' and 'xlink:href' are transferred to the generated 'g' element. An additional transformation translate(x,y) is appended to the end (i.e., right-side) of the 'transform' attribute on the generated 'g', where x and y represent the values of the 'x' and 'y' attributes on the 'use' element. The referenced 'symbol' and its contents are deep-cloned into the generated tree, with the exception that the 'symbol' is replaced by an 'svg'. This generated 'svg' will always have explicit values for attributes 'width' and 'height'. If attributes 'width' and/or 'height' are provided on the 'use' element, then these attributes will be transferred to the generated 'svg'. If attributes 'width' and/or 'height' are not specified, the generated 'svg' element will use values of '100%' for these attributes.
  
  
• If the 'use' element references an 'svg' element:
  
  In the generated content, the 'use' will be replaced by 'g', where all attributes from the 'use' element except for 'x', 'y', 'width', 'height' and 'xlink:href' are transferred to the generated 'g' element. An additional transformation translate(x,y) is appended to the end (i.e., right-side) of the 'transform' attribute on the generated 'g', where x and y represent the values of the 'x' and 'y' attributes on the 'use' element. The referenced 'svg' and its contents are deep-cloned into the generated tree. If attributes 'width' and/or 'height' are provided on the 'use' element, then these values will override the corresponding attributes on the 'svg' in the generated tree.
  
  
• Otherwise:
  
  In the generated content, the 'use' will be replaced by 'g', where all attributes from the 'use' element except for 'x', 'y', 'width', 'height' and 'xlink:href' are transferred to the generated 'g' element. An additional transformation translate(x,y) is appended to the end (i.e., right-side) of the 'transform' attribute on the generated 'g', where x and y represent the values of the 'x' and 'y' attributes on the 'use' element. The referenced object and its contents are deep-cloned into the generated tree.

For user agents that support Styling with CSS, the generated 'g' element carries along with it the "cascaded" property values on the 'use' element which result from the CSS cascade ([CSS2-CASCADE], chapter 6). Additionally, the copy (deep clone) of the referenced resource carries along with it the "cascaded" property values resulting from the CSS cascade on the original (i.e., referenced) elements. Thus, the result of various CSS selectors in combination with the 'class' and 'style' attributes are, in effect, replaced by the functional equivalent of a 'style' attribute in the generated content which conveys the "cascaded" property values.

Example Use01 below has a simple 'use' on a 'rect'.

<?xml version="1.0" standalone="no"?>
<!DOCTYPE svg PUBLIC "-//W3C//DTD SVG 1.1//EN" 
  "http://www.w3.org/Graphics/SVG/1.1/DTD/svg11.dtd">
<svg width="10cm" height="3cm" viewBox="0 0 100 30" version="1.1"
     xmlns="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink">
  <desc>Example Use01 - Simple case of 'use' on a 'rect'</desc>
  <defs>
    <rect id="MyRect" width="60" height="10"/>
  </defs>
  <rect x=".1" y=".1" width="99.8" height="29.8"
        fill="none" stroke="blue" stroke-width=".2" />
  <use x="20" y="10" xlink:href="#MyRect" />
</svg>

Example Use01

View this example as SVG (SVG-enabled browsers only)  

The visual effect would be equivalent to the following document:

<?xml version="1.0" standalone="no"?>
<!DOCTYPE svg PUBLIC "-//W3C//DTD SVG 1.1//EN" 
  "http://www.w3.org/Graphics/SVG/1.1/DTD/svg11.dtd">
<svg width="10cm" height="3cm" viewBox="0 0 100 30"
     xmlns="http://www.w3.org/2000/svg" version="1.1">
  <desc>Example Use01-GeneratedContent - Simple case of 'use' on a 'rect'</desc>
  <!-- 'defs' section left out -->

  <rect x=".1" y=".1" width="99.8" height="29.8"
        fill="none" stroke="blue" stroke-width=".2" />

  <!-- Start of generated content. Replaces 'use' -->
  <g transform="translate(20,10)">
    <rect width="60" height="10"/>
  </g>
  <!-- End of generated content -->

</svg>

View this example as SVG (SVG-enabled browsers only)  

Example Use02 below has a 'use' on a 'symbol'.

<?xml version="1.0" standalone="no"?>
<!DOCTYPE svg PUBLIC "-//W3C//DTD SVG 1.1//EN" 
  "http://www.w3.org/Graphics/SVG/1.1/DTD/svg11.dtd">
<svg width="10cm" height="3cm" viewBox="0 0 100 30" version="1.1"
     xmlns="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink">
  <desc>Example Use02 - 'use' on a 'symbol'</desc>
  <defs>
    <symbol id="MySymbol" viewBox="0 0 20 20">
      <desc>MySymbol - four rectangles in a grid</desc>
      <rect x="1" y="1" width="8" height="8"/>
      <rect x="11" y="1" width="8" height="8"/>
      <rect x="1" y="11" width="8" height="8"/>
      <rect x="11" y="11" width="8" height="8"/>
    </symbol>
  </defs>
  <rect x=".1" y=".1" width="99.8" height="29.8"
        fill="none" stroke="blue" stroke-width=".2" />
  <use x="45" y="10" width="10" height="10" 
       xlink:href="#MySymbol" />
</svg>

Example Use02

View this example as SVG (SVG-enabled browsers only)  

The visual effect would be equivalent to the following document:

<?xml version="1.0" standalone="no"?>
<!DOCTYPE svg PUBLIC "-//W3C//DTD SVG 1.1//EN" 
  "http://www.w3.org/Graphics/SVG/1.1/DTD/svg11.dtd">
<svg width="10cm" height="3cm" viewBox="0 0 100 30"
     xmlns="http://www.w3.org/2000/svg" version="1.1">
  <desc>Example Use02-GeneratedContent - 'use' on a 'symbol'</desc>

  <!-- 'defs' section left out -->

  <rect x=".1" y=".1" width="99.8" height="29.8"
        fill="none" stroke="blue" stroke-width=".2" />

  <!-- Start of generated content. Replaces 'use' -->
  <g transform="translate(45, 10)" >
    <!-- Start of referenced 'symbol'. 'symbol' replaced by 'svg',
         with x,y,width,height=0,0,100%,100% -->
    <svg width="10" height="10" 
         viewBox="0 0 20 20">
      <rect x="1" y="1" width="8" height="8"/>
      <rect x="11" y="1" width="8" height="8"/>
      <rect x="1" y="11" width="8" height="8"/>
      <rect x="11" y="11" width="8" height="8"/>
    </svg>
    <!-- End of referenced symbol -->
  </g>
  <!-- End of generated content -->

</svg>

View this example as SVG (SVG-enabled browsers only)  

Example Use03 illustrates what happens when a 'use' has a 'transform' attribute.

<?xml version="1.0" standalone="no"?>
<!DOCTYPE svg PUBLIC "-//W3C//DTD SVG 1.1//EN" 
  "http://www.w3.org/Graphics/SVG/1.1/DTD/svg11.dtd">
<svg width="10cm" height="3cm" viewBox="0 0 100 30" version="1.1"
     xmlns="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink">
  <desc>Example Use03 - 'use' with a 'transform' attribute</desc>
  <defs>
    <rect id="MyRect" x="0" y="0" width="60" height="10"/>
  </defs>
  <rect x=".1" y=".1" width="99.8" height="29.8"
        fill="none" stroke="blue" stroke-width=".2" />
  <use xlink:href="#MyRect"
       transform="translate(20,2.5) rotate(10)" />
</svg>

Example Use03

View this example as SVG (SVG-enabled browsers only)  

The visual effect would be equivalent to the following document:

<?xml version="1.0" standalone="no"?>
<!DOCTYPE svg PUBLIC "-//W3C//DTD SVG 1.1//EN" 
  "http://www.w3.org/Graphics/SVG/1.1/DTD/svg11.dtd">
<svg width="10cm" height="3cm" viewBox="0 0 100 30"
     xmlns="http://www.w3.org/2000/svg" version="1.1">
  <desc>Example Use03-GeneratedContent - 'use' with a 'transform' attribute</desc>

  <!-- 'defs' section left out -->

  <rect x=".1" y=".1" width="99.8" height="29.8"
        fill="none" stroke="blue" stroke-width=".2" />

  <!-- Start of generated content. Replaces 'use' -->
  <g transform="translate(20,2.5) rotate(10)">
    <rect x="0" y="0" width="60" height="10"/>
  </g>
  <!-- End of generated content -->

</svg>

View this example as SVG (SVG-enabled browsers only)  

Example Use04 illustrates a 'use' element with various methods of applying CSS styling.

<?xml version="1.0" standalone="no"?>
<!DOCTYPE svg PUBLIC "-//W3C//DTD SVG 1.1//EN" 
  "http://www.w3.org/Graphics/SVG/1.1/DTD/svg11.dtd">
<svg width="12cm" height="3cm" viewBox="0 0 1200 300" version="1.1"
     xmlns="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink">
  <desc>Example Use04 - 'use' with CSS styling</desc>
  <defs style=" /* rule 9 */ stroke-miterlimit: 10" >
    <path id="MyPath" d="M300 50 L900 50 L900 250 L300 250"
                     class="MyPathClass"
                     style=" /* rule 10 */ stroke-dasharray:300,100" />
  </defs>
  <style type="text/css">
    <![CDATA[
      /* rule 1 */ #MyUse { fill: blue }
      /* rule 2 */ #MyPath { stroke: red }
      /* rule 3 */ use { fill-opacity: .5 }
      /* rule 4 */ path { stroke-opacity: .5 }
      /* rule 5 */ .MyUseClass { stroke-linecap: round }
      /* rule 6 */ .MyPathClass { stroke-linejoin: bevel }
      /* rule 7 */ use > path { shape-rendering: optimizeQuality }
      /* rule 8 */ g > path { visibility: hidden }
    ]]>
  </style>

  <rect x="0" y="0" width="1200" height="300"
         style="fill:none; stroke:blue; stroke-width:3"/>
  <g style=" /* rule 11 */ stroke-width:40">
    <use id="MyUse" xlink:href="#MyPath" 
         class="MyUseClass"
         style="/* rule 12 */ stroke-dashoffset:50" />
  </g>
</svg>

Example Use04

View this example as SVG (SVG-enabled browsers only)  

The visual effect would be equivalent to the following document. Observe that some of the style rules above apply to the generated content (i.e., rules 1-6, 10-12), whereas others do not (i.e., rules 7-9). The rules which do not affect the generated content are:

• Rules 7 and 8: CSS selectors only apply to the formal document tree, not on the generated tree; thus, these selectors will not yield a match.
• Rule 9: The generated tree only inherits from the ancestors of the 'use' element and does not inherit from the ancestors of the referenced element; thus, this rule does not affect the generated content.

In the generated content below, the selectors that yield a match have been transferred into inline 'style' attributes for illustrative purposes.

<?xml version="1.0" standalone="no"?>
<!DOCTYPE svg PUBLIC "-//W3C//DTD SVG 1.1//EN" 
  "http://www.w3.org/Graphics/SVG/1.1/DTD/svg11.dtd">
<svg width="12cm" height="3cm" viewBox="0 0 1200 300"
     xmlns="http://www.w3.org/2000/svg" version="1.1">
  <desc>Example Use04-GeneratedContent - 'use' with a 'transform' attribute</desc>

  <!-- 'style' and 'defs' sections left out -->

  <rect x="0" y="0" width="1200" height="300"
         style="fill:none; stroke:blue; stroke-width:3"/>
  <g style="/* rule 11 */ stroke-width:40">

    <!-- Start of generated content. Replaces 'use' -->
    <g style="/* rule 1 */ fill:blue;
                 /* rule 3 */ fill-opacity:.5;
                 /* rule 5 */ stroke-linecap:round;
                 /* rule 12 */ stroke-dashoffset:50" >
      <path d="M300 50 L900 50 L900 250 L300 250"
            style="/* rule 2 */ stroke:red; 
                   /* rule 4 */ stroke-opacity:.5; 
                   /* rule 6 */ stroke-linejoin: bevel; 
                   /* rule 10 */ stroke-dasharray:300,100" />
    </g> 
    <!-- End of generated content -->

  </g>
</svg>

View this example as SVG (SVG-enabled browsers only)  

When a 'use' references another element which is another 'use' or whose content contains a 'use' element, then the deep cloning approach described above is recursive. However, a set of references that directly or indirectly reference a element to create a circular dependency is an error, as described in References and the 'defs' element.  

<!ENTITY % SVG.use.extra.content "" >
<!ENTITY % SVG.use.element "INCLUDE" >
<![%SVG.use.element;[
<!ENTITY % SVG.use.content
    "(( %SVG.Description.class; )*, ( %SVG.Animation.class;
        %SVG.use.extra.content; )*)"
>
<!ELEMENT %SVG.use.qname; %SVG.use.content; >
<!-- end of SVG.use.element -->]]>
<!ENTITY % SVG.use.attlist "INCLUDE" >
<![%SVG.use.attlist;[
<!ATTLIST %SVG.use.qname;
    %SVG.Core.attrib;
    %SVG.Conditional.attrib;
    %SVG.Style.attrib;
    %SVG.Presentation.attrib;
    %SVG.GraphicalEvents.attrib;
    %SVG.XLinkEmbed.attrib;
    %SVG.External.attrib;
    x %Coordinate.datatype; #IMPLIED
    y %Coordinate.datatype; #IMPLIED
    width %Length.datatype; #IMPLIED
    height %Length.datatype; #IMPLIED
    transform %TransformList.datatype; #IMPLIED
>

5.7 The 'image' 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.

The result of processing an 'image' is always a four-channel RGBA result. When an 'image' element references a raster image file such as 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 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.

An 'image' element establishes a new viewport for the referenced file as described in Establishing a new viewport.  The bounds for the new viewport are defined by attributes 'x', 'y', 'width' and 'height'. The placement and scaling of the referenced image are controlled by the 'preserveAspectRatio' attribute on the 'image' element.

When an 'image' element references an SVG image the 'preserveAspectRatio' attribute as well as the clip and overflow properties on the root element in the referenced SVG image are ignored (in the same manner as the x, y, width and height attributes are ignored). Instead, the 'preserveAspectRatio' attribute on the referencing 'image' element defines how the SVG image content is fitted into the viewport and the clip and overflow properties on the 'image' element define how the SVG image content is clipped (or not) relative to the viewport.

The value of the 'viewBox' attribute to use when evaluating the 'preserveAspectRatio' attribute is defined by the referenced content. For content that clearly identifies a viewBox (e.g. an SVG file with the 'viewBox' attribute on the outermost svg element) that value should be used. For most raster content (PNG, JPEG) the bounds of the image should be used (i.e. the 'image' element has an implicit 'viewBox' of "0 0 raster-image-width raster-image-height"). Where no value is readily available (e.g. an SVG file with no 'viewBox' attribute on the outermost 'svg' element) the 'preserveAspectRatio' attribute is ignored, and only the translate due to the 'x' & 'y' attributes of the viewport is used to display the content.

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 fitted 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').

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.  

<!ENTITY % SVG.Image.extra.class "" >
<!ENTITY % SVG.Image.class
    "| %SVG.image.qname; %SVG.Image.extra.class;"
>
<!-- image: Image Element .............................. -->
<!ENTITY % SVG.image.extra.content "" >
<!ENTITY % SVG.image.element "INCLUDE" >
<![%SVG.image.element;[
<!ENTITY % SVG.image.content
    "(( %SVG.Description.class; )*, ( %SVG.Animation.class;
        %SVG.image.extra.content; )*)"
>
<!ELEMENT %SVG.image.qname; %SVG.image.content; >
<!-- end of SVG.image.element -->]]>
<!ENTITY % SVG.image.attlist "INCLUDE" >
<![%SVG.image.attlist;[
<!ATTLIST %SVG.image.qname;
    %SVG.Core.attrib;
    %SVG.Conditional.attrib;
    %SVG.Style.attrib;
    %SVG.Viewport.attrib;
    %SVG.Color.attrib;
    %SVG.Opacity.attrib;
    %SVG.Graphics.attrib;
    %SVG.ColorProfile.attrib;
    %SVG.Clip.attrib;
    %SVG.Mask.attrib;
    %SVG.Filter.attrib;
    %SVG.GraphicalEvents.attrib;
    %SVG.Cursor.attrib;
    %SVG.XLinkEmbed.attrib;
    %SVG.External.attrib;
    x %Coordinate.datatype; #IMPLIED
    y %Coordinate.datatype; #IMPLIED
    width %Length.datatype; #REQUIRED
    height %Length.datatype; #REQUIRED
    preserveAspectRatio %PreserveAspectRatioSpec.datatype; 'xMidYMid meet'
    transform %TransformList.datatype; #IMPLIED
>

An example:

<?xml version="1.0" standalone="no"?>
<!DOCTYPE svg PUBLIC "-//W3C//DTD SVG 1.1//EN" 
  "http://www.w3.org/Graphics/SVG/1.1/DTD/svg11.dtd">
<svg width="4in" height="3in" version="1.1"
     xmlns="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink">
  <desc>This graphic links to an external image
  </desc>
  <image x="200" y="200" width="100px" height="100px"
         xlink:href="myimage.png">
    <title>My image</title>
  </image>
</svg>