Chapter 16: Linking

16.1. References

16.1.1. Overview

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

16.1.2. 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.

external file reference

A URL reference or URL reference with fragment identifier which refers to a resource that is not part of the current document.

same-document URL reference

A URL reference with fragment identifier where the non-fragment part of the URL refers to the current document.

data URL

A URL reference to an embedded document specified using the "data" URL scheme [rfc2397]. Data URL references are neither external file references nor same-document URL references.

circular reference

URL references that directly or indirectly reference themselves are treated as invalid circular references. What constitutes a circular reference will depend on how the referenced resource is used, and may include a reference to an ancestor of the current element.

unresolved reference

A reference that is still being processed, and has not yet resulted in either an error or an identified resource.

invalid reference

Any of the following are invalid references:

• A circular reference.
• A URL reference that results in an error during processing.
• A URL reference that cannot be resolved.
• A URL references to elements which are inappropriate targets for the given reference shall be treated as invalid references (see Valid URL targets 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’.

Invalid references may or may not be an error (see Error processing), depending on whether the referencing property or attribute defines fallback behavior.

16.1.3. 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 [xmlschema-2]. 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.

16.1.4. Syntactic forms: URL and <url>

In SVG, most structural relationships between two elements are specified using a URL value in an ‘href’ attribute. However, many presentation attributes allow both URLs and text strings as content. To disambiguate a text string from a relative URL, the <url> production is used for presentation attributes, and their corresponding CSS properties [css-values]. This is simply a URL delimited with a functional notation.

SVG makes extensive use of URL references, both absolute and relative, to other objects. For example, a ‘linearGradient’ element may be based on another gradient element, so that only the differences between the two need to be specified, by referencing the source gradient with a URL in the ‘href’ attribute:

<linearGradient id="SourceGradient">...</linearGradient>
<linearGradient id="MyGradient"
                href="#SourceGradient">...</linearGradient>

To fill a rectangle with that gradient, the value of the rectangle's fill property may be set so as to include a URL reference to the relevant ‘linearGradient’ element; here is an example:

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

16.1.5. 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 authoring tools. An invalid URL does not make an SVG document non-conforming. SVG user agents are only required to process URLs when needed, as specified in Processing of URL references.

16.1.6. 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 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-names], 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>

16.1.7. Processing of URL references

URLs are processed to identify a resource at the time they are needed, as follows:

• For the ‘href’ attribute of the ‘a’ element, at the time the link is activated by the user.
• For all other ‘href’ attributes, at the time the element is connected to a document, or at the time when the attribute is set, whichever is later.
• For source URLs on embedded HTML media elements, as required based on source selection rules in the HTML specification [HTML].
• For all presentation attributes and style properties, at the time the property is required for rendering an element.

Legacy ‘xlink:href’ attributes are processed at the time a corresponding ‘href’ attribute would be processed, but only if no such ‘href’ attribute exists on the element.

Processing a URL involves three steps: generating the absolute URL; fetching the document (if required); identifying the target element (if required).

A URL reference is unresolved until processing either results in an invalid reference or in the identification of the target resource. Unresolved references in the non-presentation attributes of structurally external elements prevent the load event from firing. User agents may place time limits on the resolution of references that are not same-document URL references, after which the reference is treated as a network error (and therefore as an invalid reference).

For same-document URL references in a dynamic document, modifications or animations of attributes or properties, or removal of elements from the DOM, may cause an URL reference to return to the unresolved state. The user agent must once again attempt to resolve the URI to identify the referenced resource.

16.1.7.1. Generating the absolute URL

If the URL reference is relative, its absolute version must be computed before use. The absolute URL should be generated using one of the following methods:

• as described in CSS Values and Units if the reference occurs in a CSS file or in a presentation attribute [css-values]
• as described in XML Base before use [xmlbase] if the reference occurs in any other attribute in an XML document
• as described in the HTML specification if the reference occurs in a non-presentation attribute in an HTML document that is not an XML document [HTML]

The ‘xml:base’ attribute will only have an effect in XML documents; this includes SVG documents and XHTML documents but not HTML documents that are not XML. In contrast, a base element affects relative URLs in any SVG or HTML document, by altering the document base URL.

If the protocol, such as HTTP, does not support URLs directly, the URL must be converted to a URI by the user agent, as described in section 3.1 of the URL specification [rfc3987].

After generating the absolute URL:

• If the URL is being processed following the activation of a link, the user agent must follow the algorithm for navigating to a URL described in the HTML specification [HTML]. The outcome of this algorithm varies depending on the ‘target’ browsing context and security restrictions between browsing contexts, and on whether the link is to the same document as is currently contained in that browsing context (in which case the fragment is navigated without reloading the document). If the document that was navigated was an SVG document, then adjust the target behavior as described in Linking into SVG content.

• If the URL being processed is only valid if it refers to a complete document file (such as the ‘href’ attribute of an ‘image’ and ‘script’ element), continue as indicated in Fetching the document (regardless of whether the URL is to the same document or not).

• In all other cases, the URL is for a resource to be used in this SVG document. The user agent must parse the URL to separate out the target fragment from the rest of the URL, and compare it with the document base URL. If all parts other than the target fragment are equal, this is a same-document URL reference, and processing the URL must continue as indicated in Identifying the target element with the current document as the referenced document.

• Otherwise, the URL references a separate document, and the user agent must continue processing the URL as indicated in Fetching the document.

As defined in CSS Values and Units, a fragment-only URL in a style property must be treated as a same-document URL reference, regardless of the file in which the property was declared.

16.1.7.2. Fetching the document

SVG properties and attributes may reference other documents. When processing such a URL, the user agent should fetch the referenced document as described in this section, except under the following conditions:

• If the URL reference is from the href attribute on SVG animation elements, only same-document URL references are allowed [svg-animation]. A URL referring to a different document is invalid and the document must not be fetched.

• If the document containing the reference is being processed in secure static mode or secure animated mode, external file references are disallowed. Unless the reference is a data URL, the user agent must treat the reference as if there was a network error, making this an invalid reference.

• If any other security restrictions on the browsing context or user agent prevent accessing the external file, then the user agent must treat the reference as if there was a network error.

When fetching external resources from the Internet, user agents must use a potentially CORS-enabled request as defined in HTML [HTML] with the corsAttributeState as follows:

• For an ‘href’ reference on an ‘image’ element or ‘script’ element, the CORS state specified by the ‘crossorigin’ attribute.
• For a reference from a style property or presentation attribute, the "anonymous" state.
• For all other references, the "no-cors" state.

The request's origin is computed using the same rules as HTML, with an SVG ‘script’ element treated like an HTML script element, and an SVG ‘image’ element treated like an HTML img element. The default origin behaviour must be set to taint.

A future SVG specification may enable CORS references on other SVG elements with ‘href’ attributes.

If the fetching algorithm results in an error or an empty response body, the reference URL is treated as an invalid reference.

If a valid response is returned, and the valid URL targets for the reference include specific element types, the user agent must continue by Processing the subresource document. Otherwise (if only entire-document the URL references are valid), then the fetched document is the referenced resource.

16.1.7.3. Processing the subresource document

Otherwise, the subresource must be parsed to identify the target element. If the fetched document is a type that the user agent can parse to create a document object model, it must process it in secure static mode (meaning, do not fetch any additional external resources and do not run scripts or play animations or video). The document model generated for an external subresource reference must be immutable (read-only) and cannot be modified.

If a document object model can be generated from the fetched file, processing the URL must continue as indicated in Identifying the target element with the parsed subresource document as the referenced document. The user agent may commence the target-identification process prior to completely parsing the document.

User agents may maintain a list of external resource URLs and their associated parsed documents, and may re-use the documents for subsequent references, so long as doing so does not violate the processing mode, caching, and CORS requirements on the resource.

16.1.7.4. Identifying the target element

For URL references to a specific element, whether the reference is valid depends on whether the element can be located within the referenced document and whether it is of an allowed type.

Using the referenced document identified in previous processing steps (either an external subresource document or the current document), the target element is identified as follows:

• If the URL does not specify a specific element in a target fragment, the target element is the root element of the referenced document.

• Otherwise, the URL targets a specific element. If a matching element currently exists in the referenced document, then it is the target element.

• Otherwise, there is no currently matching element. If the referenced document is immutable, then the URL reference is invalid. An external subresource document is always immutable once fully parsed; the current document is also immutable once parsed if it is being processed in any mode other than dynamic interactive mode.

• Otherwise, observe mutations to the referenced document until the URL can be successfully resolved to define a target element, or until the document becomes immutable (e.g., a non-dynamic document finishes parsing).

The target element provides the referenced resource if (and only if) it is a valid URL target for the reference.

16.1.7.5. Valid URL targets

The valid target element types for ‘href’ (or ‘xlink:href’) attributes are based on the element that has the attribute, as follows:

• the ‘a’ element can reference any local or non-local resource
• the ‘cursor’ element must reference a document that can be processed as an image
• the ‘hatch’ element must reference another ‘hatch’ element
• the ‘image’ element must reference a document that can be processed as an image
• the ‘linearGradient’ element must reference a different ‘linearGradient’ or ‘radialGradient’ element
• the ‘meshgradient’ element must reference a different ‘meshgradient’ element
• the ‘mesh’ element must reference a ‘meshgradient’ element
• the ‘pattern’ element must reference another ‘pattern’ element
• the ‘radialGradient’ element must reference a ‘linearGradient’ or another ‘radialGradient’ element
• the ‘script’ element must reference an external document that provides the script content
• the ‘textPath’ element must reference an element type that implements the SVGGeometryElement interface
• the ‘use’ element must reference either an SVG-namespaced element or an HTML-namespaced element that may be included as a child of an SVG container element

The valid target element types for style properties defined in this specification are as follows:

• the cursor property must reference either a ‘cursor’ element or a document that can be processed as an image
• the fill property (see Specifying paint for reference rules)
• the marker, marker-start, marker-mid and marker-end properties must reference a ‘marker’ element.
• the shape-inside and shape-subtract properties must reference an element type that implements the SVGGeometryElement interface, or a document that can be processed as an image
• the stroke property (see Specifying paint for reference rules)

For references that allow either a reference to a target element, or to an image file (such as the cursor, shape-inside, shape-subtract, and mask properties), the user agent must identify the target element and determine whether it is a valid target. If the resolved target element is not an allowed element type, the referenced resource is the entire document file; the target fragment is used in processing that file as with any other image.

In all other cases, if the resolved target element type (or document type) is not allowed for the URL reference, it is an invalid reference.