While a human may be able to read the explanatory text in order to figure out which links point to schemas and which links point to stylesheets, software needs somewhat more formal hints to ensure that a validator doesn't try to validate a document against a stylesheet or that a browser doesn't try to format a document as described by a DTD. To this end, the xlink:role attribute may contain a URL that indicates the nature of the resource likely to be found at the other end of the simple link. For example, this rddl:resource element locates a W3C XML Schema Language schema:

 <rddl:resource xlink:type="simple"    xlink:href=""    xlink:role="" /> 

This rddl:resource element locates a DTD:

 <rddl:resource xlink:type="simple"    xlink:href=""    xlink:role=" notes/iana/assignments/media-types/application/xml-dtd" /> 

This rddl:resource element locates a CSS stylesheet:

 <rddl:resource xlink:type="simple"    xlink:href=""    xlink:role=" -notes/iana/assignments/media-types/text/css" /> 

As in these examples, most nature URIs are derived from either XML namespace URLs or MIME media type URLs. The RDDL specification currently defines the 22 roles listed in Table 42-1. However, new ones are added regularly; you are free to invent and use your own. Most software will silently ignore any rddl:resource elements with unfamiliar natures.

Table 42-1. RDDL Natures



CSS stylesheet



HTML 4 document

Strict HTML 4 document

HTML 4 transitional document

HTML 4 Frameset document

XHTML document

XHTML 1.0 strict document

XHTML 1.0 transitional document

Resource Description Framework Schema [a]

XML Schema module

RELAX schema [b]

RELAX core grammar

RELAX namespace grammar

Schematron schema

An OASIS Open Catalog [c]

XML character data

Escaped XML text

Unparsed binary data


An ISO standard

[a] Not the same thing as a W3C XML Schema Language schema.

[b] RELAX is a precursor to RELAX NG. A RELAX NG nature is a likely candidate for the future.

[c] See Item 47.

Effective XML. 50 Specific Ways to Improve Your XML
Effective XML: 50 Specific Ways to Improve Your XML
ISBN: 0321150406
EAN: 2147483647
Year: 2002
Pages: 144 © 2008-2017.
If you may any questions please contact us: