Document Type Definition
The DTD specifies the logical structure of the document; it is a formal grammar describing document syntax and semantics
The DTD does not describe the physical layout of the document; this is left to the style sheets and the scripts
It is no mean task to write a DTD, so most users will adopt predefined DTDs
DTDs can be written in separate files to facilitate re-use.
Content-providers, industries and other groups can collaborate to define sets of tags.