Extensibility has released Data Types for DTDs (DT4DTD), with both a specification and a Java implementation for SAX and the DOM.

DT4DTD puts the techniques Extensibility used for data typing with DTDs in their XML Authority product on a firmer and more public footing. Using a combination of attribute and notation declarations, DT4DTD allows DTD designers to specify data types for element and attribute content using data typing mechanisms from the XML Schemas Datatypes proposals under development at the W3C. Developers can also extend the system with their own data types.

The Java implementation allows developers to add a layer of data type checking between their SAX or DOM parser's output and the application, allowing developers to shift this burden from their own application layers to the generic DT4DTD layer. This approach effectively makes it possible to retrofit SAX and DOM parsers for data type support.

XML Authority can convert schemas to DTDs and preserve data type information using this approach, giving developers weary of the cutting edge a wider range of tools for processing. XML Authority can also convert DTDs that use this approach to schemas of a wide variety of types, making DT4DTD a possible upgrade path toward eventual schema usage.