Chapter 15: Linking

Contents

• • 15.1. References
    • 15.1.1. Overview
    • 15.1.2. Definitions
    • 15.1.3. URLs and URIs
    • 15.1.4. Syntactic forms: URL and <url>
    • 15.1.5. Processing of URL references
    • 15.1.6. URL reference attributes
    • 15.1.7. Deprecated XLink URL reference attributes
  • 15.2. Links out of SVG content: the ‘a’ element
  • 15.3. Linking into SVG content: URL fragments and SVG views
    • 15.3.1. SVG fragment identifiers
    • 15.3.2. SVG fragment identifiers definitions
    • 15.3.3. Predefined views: the ‘view’ element
  • 15.4. DOM interfaces
    • 15.4.1. Interface SVGAElement
    • 15.4.2. Interface SVGViewElement

15.1. References

15.1.1. Overview

Make sure all other chapters refer to href and not xlink:href.

On the Internet, resources are identified using URLs (Internationalized Resource Identifiers). For example, an SVG file called someDrawing.svg located at http://example.com might have the following URL:

http://example.com/someDrawing.svg

An URL can also address a particular element within an XML document by including an URL fragment identifier as part of the URL. An URL which includes an URL fragment identifier consists of an optional base URL, followed by a "#" character, followed by the URL fragment identifier. For example, the following URL can be used to specify the element whose ID is "Lamppost" within file someDrawing.svg:

http://example.com/someDrawing.svg#Lamppost

15.1.2. Definitions

URL reference

An URL reference is an Internationalized Resource Identifier, as defined in Internationalized Resource Identifiers [RFC3987]. See References and References and the ‘defs’ element.

URL reference with fragment identifier

An Internationalized Resource Identifier [RFC3987] that can include an <absoluteURL> or <relativeURL> and a identifier of the fragment in that resource. See References and the ‘defs’ element. URL reference with fragment identifiers are commonly used to reference paint servers.

circular reference

URL references that directly or indirectly reference themselves are treated as invalid circular references.

invalid reference

Either of the following:

• URL reference to node that do not exist.
• URL references to elements which are inappropriate targets for the given reference shall be treated as invalid references (see Processing of URL references 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’.

15.1.3. URLs and URIs

Internationalized Resource Identifiers (URLs) are a more generalized complement to Uniform Resource Identifiers (URIs). An URL is a sequence of characters from the Universal Character Set [UNICODE]. A URI is constructed from a much more restricted set of characters. All URIs are already conformant URLs. A mapping from URLs to URIs is defined by the URL specification, which means that URLs can be used instead of URIs in XML documents, to identify resources. URLs can be converted to URIs for resolution on a network, if the protocol does not support URLs directly.

Previous versions of SVG, following XLink, defined an URL reference type as a URI or as a sequence of characters which must result in an URL after a particular escaping procedure was applied. The escaping procedure was repeated in the XLink 1.0 specification [XLINK], and in the W3C XML Schema Part 2: Datatypes specification [SCHEMA2]. This copying introduced the possibility of error and divergence, but was done because the URL specification was not yet standardized.

In this specification, the correct term URL is used for this "URI or sequence of characters plus an algorithm" and the escaping method, which turns URLs into URIs, is defined by reference to the URL specification [RFC3987], which has since become an IETF Proposed Standard. Other W3C specifications are expected to be revised over time to remove these duplicate descriptions of the escaping procedure and to refer to URL directly.

15.1.4. Syntactic forms: URL and <url>

In most cases, URLs are specified using an ‘href’ attribute. However, some attributes allow both URLs and text strings as content. To disambiguate a text string from a relative URL, the <url> production is used [CSS3VALUES]. This is simply a URL delimited with a functional notation. This form is used in presentation attributes.

SVG makes extensive use of URL references, both absolute and relative, 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 the following example:

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

15.1.5. Processing of URL references

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

• the ‘a’ element can reference any local or non-local resource
• the ‘animate’ 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 ‘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 ‘fill’ property (see Specifying paint for reference rules)
• 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 ‘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 ‘textPath’ element must reference a ‘path’ 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 URL references:

• An invalid local URL reference (i.e., an invalid references to a node within the current document) represents an error (see Error processing), apart from the ‘href’ attribute on the ‘a’ element and the properties that allow for backup values in the case where the URL reference is invalid (see ‘fill’ and ‘stroke’).
• An invalid circular URL reference represents an error (see Error processing).

15.1.6. URL reference attributes

URL references are normally specified with an ‘href’ attribute. The value of this attribute forms a reference for the desired resource (or secondary resource, if there is a fragment identifier). The value of the ‘href’ attribute must be a URL.

Because it is impractical for any application to check that a value is an URL reference, this specification follows the lead of the URL Specification in this matter and imposes no such conformance testing requirement on SVG applications.

If the URL reference is relative, its absolute version must be computed by the method described in XML Base before use [XML-BASE].

If the protocol, such as HTTP, does not support URLs directly, the URL is converted to a URI by the SVG implementation, as described in section 3.1 of the URL specification [RFC3987].

15.1.7. Deprecated XLink URL reference attributes

In previous versions of SVG, the ‘href’ attribute was specified in the XLink namespace [XLINK] namespace. This usage is now deprecated and instead URL references should be specified using the ‘href’ attribute without a namespace.

For backwards compatibility, the deprecated ‘xlink:href’ attribute is defined below along with the ‘xlink:title’ attribute which has also been deprecated.

Attribute definitions:

Name	Value	Initial value	Animatable
xlink:href	URL [URL]	(none)	(see below)

For backwards compatibility, elements with an ‘href’ attribute also recognize an ‘href’ attribute in the XLink namespace [XLINK].

When the ‘href’ attribute is present in both the the XLink namespace and without a namespace, the value of the attribute without a namespace shall be used. The attribute in the XLink namespace shall be ignored.

A Conforming SVG Generator must generate ‘href’ attributes without a namespace. However, it may also generate ‘href’ attributes in the XLink namespace to provide backwards compatibility.

This attribute is Animatable if and only if the corresponding ‘href’ attribute is defined to be animatable.

Name	Value	Initial value	Animatable
xlink:title	<anything>	(none)	no

Deprecated attribute to describe the meaning of a link or resource in a human-readable fashion. New content should use a ‘title’ child element rather than a ‘xlink:title’ attribute.

The use of this information is highly dependent on the type of processing being done. It may be used, for example, to make titles available to applications used by visually impaired users, or to create a table of links, or to present help text that appears when a user lets a mouse pointer hover over a starting resource.

The ‘title’ attribute, if used, must be in the XLink namespace. Refer to the XML Linking Language (XLink) [XLINK].

When using the deprecated XLink attributes ‘xlink:href’ or ‘xlink:title’ an explicit XLink namespace declaration must be provided [XML-NS], One simple way to provide such an XLink namespace declaration is to include an ‘xmlns’ attribute for the XLink namespace on the ‘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>

15.2. Links out of SVG content: the ‘a’ element

SVG provides an ‘a’ element, to indicate links (also known as hyperlinks or Web links). The ‘a’ element may contain any element that its parent may contain, except itself.

SVG uses XLink ([XLink]) for all link definitions. SVG 1.1 only requires that user agents support XLink's notion of simple links. Each simple link associates exactly two resources, one local and one remote, with an arc going from the former to the latter.

A simple link is defined for each separate rendered element contained within the ‘a’ element; thus, if the ‘a’ element contains three ‘circle’ elements, a link is created for each circle. For each rendered element within an ‘a’ element, the given rendered element is the local resource (the source anchor for the link).

The remote resource (the destination for the link) is defined by an URL specified by the ‘href’ attribute on the ‘a’ element. The remote resource may be any Web resource (e.g., an image, a video clip, a sound bite, a program, another SVG document, an HTML document, an element within the current document, an element within a different document, etc.). By activating these links (by clicking with the mouse, through keyboard input, voice commands, etc.), users may visit these resources.

Example link01 assigns a link to an ellipse.

<?xml version="1.0" standalone="no"?>
<svg width="5cm" height="3cm" viewBox="0 0 5 3" version="1.1"
     xmlns="http://www.w3.org/2000/svg">
  <desc>Example link01 - a link on an ellipse
  </desc>
  <rect x=".01" y=".01" width="4.98" height="2.98" 
        fill="none" stroke="blue"  stroke-width=".03"/>
  <a href="http://www.w3.org">
    <ellipse cx="2.5" cy="1.5" rx="2" ry="1"
             fill="red" />
  </a>
</svg>

Example link01

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

If the above SVG file is viewed by a user agent that supports both SVG and HTML, then clicking on the ellipse will cause the current window or frame to be replaced by the W3C home page.

Attribute definitions:

Name	Value	Initial value	Animatable
href	URL [URL]	(none)	yes

The location of the referenced object, expressed as an URL reference. Refer to the common handling defined for URL reference attributes.

Name	Value	Initial value	Animatable
target	_self | _parent | _top | _blank | <XML-Name>	_self	yes

This attribute should be used when there are multiple possible targets for the ending resource, such as when the parent document is embedded within an HTML or XHTML document, or is viewed with a tabbed browser. This attribute specifies the name of the browsing context (e.g., a browser tab or an SVG, HTML, or XHTML iframe or object element) into which a document is to be opened when the link is activated:

_self

The current SVG image is replaced by the linked content in the same browsing context as the current SVG image.

_parent

The immediate parent browsing context of the SVG image is replaced by the linked content, if it exists and can be securely accessed from this document.

_top

The content of the full active window or tab is replaced by the linked content, if it exists and can be securely accessed from this document

_blank

A new un-named window or tab is requested for the display of the linked content, if this document can securely do so. If the user agent does not support multiple windows/tabs, the result is the same as _top.

<XML-Name>

Specifies the name of the browsing context (tab, inline frame, object, etc.) for display of the linked content. If a context with this name already exists, and can be securely accessed from this document, it is re-used, replacing the existing content. If it does not exist, it is created (the same as '_blank', except that it now has a name). The name must be a valid XML Name [XML11], and should not start with an underscore (U+005F LOW LINE character), to meet the requirements of a valid browsing context name from HTML.

The normative definitions for browsing contexts and security restrictions on navigation actions between browsing contexts is HTML 5 [HTML], specifically the chapter on loading web pages.

Previous versions of SVG defined the special target value '_replace'. It was never well implemented, and the distinction between '_replace' and '_self' has been made redundant by changes in the HTML definition of browsing contexts. Use '_self' to replace the current SVG document.

The value '_new' is not a legal value for target. Use '_blank' to open a document in a new tab/window.

15.3. Linking into SVG content: URL fragments and SVG views

Because SVG content often represents a picture or drawing of something, a common need is to link into a particular view of the document, where a view indicates the initial transformations so as to present a closeup of a particular section of the document.

SVGFragmentIdentifier ::= BareName *( "&" timesegment ) |
                          SVGViewSpec *( "&" timesegment ) |
                          spacesegment *( "&" timesegment ) |
                          timesegment *( "&" spacesegment )

BareName ::= XML_Name
SVGViewSpec ::= 'svgView(' SVGViewAttributes ')'
SVGViewAttributes ::= SVGViewAttribute |
                      SVGViewAttribute ';' SVGViewAttributes

SVGViewAttribute ::= viewBoxSpec |
                     preserveAspectRatioSpec |
                     transformSpec |
                     zoomAndPanSpec
viewBoxSpec ::= 'viewBox(' ViewBoxParams ')'
preserveAspectRatioSpec = 'preserveAspectRatio(' AspectParams ')'
transformSpec ::= 'transform(' TransformParams ')'
zoomAndPanSpec ::= 'zoomAndPan(' ZoomAndPanParams ')'

15.3.3. Predefined views: the ‘view’ element

The ‘view’ element is defined as follows:

15.4. DOM interfaces

interface SVGAElement : SVGGraphicsElement {
  [SameObject] readonly attribute SVGAnimatedString target;
};

SVGAElement implements SVGURIReference;

interface SVGViewElement : SVGElement;

SVGViewElement implements SVGFitToViewBox;
SVGViewElement implements SVGZoomAndPan;