Introduction
It is possible to define in advance which elements can be used within a document, and where in the document these elements may be employed. This concept is similar to, but more powerful than, the stylesheet mechanisms that all modern word processors and desktop publishing packages provide.
HTML Note
There are SGML DTDs that describe HTML 2.0, HTML 3.2 and now HTML 4.0. These DTDs are not compatible with XML, primarily because they use some of the additional features of SGML. There is now also an XML DTD for the new XHTML standard.
SGML Note
XML DTDs use the same syntax as SGML DTDs, but have a more limited scope (Chapter 32 covers the differences in detail). Also note that the XML Schema standard re-introduces many of the features ...
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