The recent publication of three W3C documents, XForms Proposed Recommendation, XML Events Proposed Recommendation, and xml:id Requirements may mark a new trend toward fewer standalone XML vocabularies and more "building block" specifications.

The need for modular XML has been evident at least since Namespaces in XML was published in 1999, though some of the more difficult issues involved are only now beginning to be addressed.

A major step forward came in 2001, with Modularization of XHTML. Despite the specific title, the framework outlined in that document is generally suitable for combining XML vocabularies of all sorts, XHTML or not. In practice, though, developers adopted this technique only to a limited degree.

More recently, an increasing percentage of W3C publications define smaller reusable parts, not monolithic vocabularies, SVG 1.1, for instance, was largely a modularization of SVG 1.0.

The XForms Proposed Recommendation, released on August 1, 2003, states:

XForms is not a free-standing document type, but is intended to be integrated into other markup languages, such as XHTML or SVG.

XML Events contains similar language, and the xml:id requirements discuss ways the eventual specification can be layered onto existing vocabluaries.

While xml:id is a welcome addition to the toolbox, developers are still coming to terms with a number of challenges associated with this trend:

• A near-combinatorial explosion of possible document types. Existing techniques, including media types like application/xhtml+xml, are not capable of fully expressing mixed document types.
• Closely related, the explosion of XML Namespace declarations needed near the top (and often in other places) in an XML document is making it nearly impossible for "hand authors" to work with XML.
• Difficulties associated with cleanly nesting one XML document in another continue to raise difficulties where DTD-association is needed (often in situations where DTDs are needed for purposes other than validation, such as defining entities).
• For hypertext applications, the discord surrounding a modular linking technology continues to hurt adoption.

Related material:

• Report on the W3C XForms Implementation Workshop
• XML Events, XHTML in W3C XML Schema
• Detailed xml:id discussion on www-tag