Document Type Definitions

Good in their day?

A mixed inheritance XML's use of SGML DTDs made it possible to use XML documents in the existing base of SGML software. At the same time, however, XML's allowing documents not to have a DTD at all makes it much easier to question the need for DTDs and to ponder very different replacements for DTDs.
A solid foundation DTDs provide an excellent set of tools for describing document structures, as well as a powerful set of tools for reusing descriptions. It's not necessarily what a developer working with OOP is used to, but they've got a lot of power.
Capable but misunderstood Between the lack of clear explanation in the specification and the fact that most users of (and writers about) XML come from outside of SGML, much of the power of DTDs goes unused. They may not fit most developers' needs perfectly, but they aren't actually that far off.