Document Definition Markup Language (DDML) is an XML schema language proposed in 1999 by various contributors from the xml-dev electronic mailing list. It was published only as a W3C Note, not a Recommendation, and never found favor with developers. An XML schema is a description of a type of XML document, typically expressed in terms of constraints on the structure and content of documents of that type, above and beyond the basic syntax constraints imposed by XML itself. ... 1999 (MCMXCIX) was a common year starting on Friday, and was designated the International Year of Older Persons by the United Nations. ... Electronic mailing lists are a special usage of email that allows for widespread distribution of information to many Internet users. ... The World Wide Web Consortium (W3C) is a consortium that produces standards—recommendations, as they call them—for the World Wide Web. ...

DDML began as XSchema, a reformulation of XML DTDs as full XML documents, so that elements and attributes, rather than declarations, could be used to describe a schema. As development continued, the name was changed to DDML, reflecting a shift away from the goal of replicating all DTD functionality, in order to concentrate on providing a robust framework for describing basic element/attribute hierarchy. DDML offered no datatypes or functionality beyond what DTDs already provided, so there was not much advantage to using DDML instead of DTDs. DDML did, however, inform the development of the next generation of XML-based schema languages, including the more successful XML Schema and RELAX NG. A Document Type Definition (DTD for short), one of several XML schema languages, is a set of declarations that conform to a particular markup syntax and that describe a class, or type, of SGML or XML documents, in terms of constraints on the structure of those documents. ... In computer science, a datatype or data type (often simply a type) is a name or label for a set of values and some operations which one can perform on that set of values. ... An XML schema is a description of a type of XML document, typically expressed in terms of constraints on the structure and content of documents of that type, above and beyond the basic syntax constraints imposed by XML itself. ... In computing, RELAX NG (REgular LAnguage for XML Next Generation) is a schema language for XML, based on Murata Makotos RELAX and James Clarks TREX. A RELAX NG schema specifies a pattern for the structure and content of an XML document. ...

External links

• The DDML specification (W3C Note)