XML Unleashed

Modeling Data with DTDs

DTDs are a means of establishing a schema for XML documents. DTDs originated in SGML and serve as the standard schema mechanism for validating SGML documents. Since XML is a subset of SGML, it only makes sense to use the same schema approach. A big benefit of using DTDs as schemas is that existing SGML tools can be easily modified to support XML because they already support DTDs in SGML. DTDs are not perfect, however. In fact, their limitations are serious enough to warrant considering another approach to modeling XML data.

DTDs rely on a specialized syntax for describing the structure of XML vocabularies. This is one of the XML community's big complaints about DTDs. Why should it be necessary to learn a specialized syntax ...