The OASIS DITA Technical Committee uses the RELAX NG XML syntax for the normative versions of the XML grammar files that comprise the DITA release.
The DITA Technical Committee chose the RELAX NG XML syntax for the following reasons:
- Easy use of foreign markup
- The DITA grammar files maintained by OASIS depend on this feature of RELAX NG in order
to capture metadata about document-type shells and modules; such metadata is used to
generate the DTD- and XSD-based versions of the grammar files.
The foreign vocabulary feature also can be used to include Schematron rules directly in RELAX NG grammars. Schematron rules can check for patterns that either are not expressible with RELAX NG directly or that would be difficult to express.
- RELAX NG div element
- This general grouping element allows for arbitrary organization and grouping of patterns within grammar documents. Such grouping tends to make the grammar documents easier to work with, especially in XML-aware editors. The use or non-use of the RELAX NG div element does not affect the meaning of the patterns that are defined in a RELAX NG schema.
- Capability of expressing precise restrictions
- RELAX NG is capable of expressing constraints that are more precise than is possible
with either DTDs or XSDs. For example, RELAX NG patterns can be context specific such that
the same element type can allow different content or attributes in different
If you plan to generate DTD- or XSD-based modules from RELAX NG modules, avoid RELAX NG features that cannot be translated into DTD or XSD constructs. When RELAX NG is used directly for DITA document validation, the document-type shells for those documents can integrate constraint modules that use the full power of RELAX NG to enforce constraints that cannot be enforced by DTDs or XSDs. The grammar files provided by the OASIS DITA Technical Committee do not use any features of RELAX NG that cannot be translated into equivalent DTD or XSD constructs.
The DITA use of RELAX NG depends on the RELAX NG DTD Compatibility specification, which provides a mechanism for defining default attribute values and embedded documentation. Processors that use RELAX NG for DITA documents in which required attributes (for example, the domains and class attributes) are not explicitly present must implement the DTD compatibility specification in order to get default attribute values.