Now that we understand basic XHTML concrete syntax and are familiar with the semantics
of a number of XHTML elements, we’re ready to see how the abstract syntax of a version
of XHTML is defined using XML. By the end of this section, you should be able to read
and understand the formal definition for any of the three flavors of XHTML 1.0.
The abstract syntax for each flavor of XHTML 1.0 is defined by a set of text files
known collectively as an XML document type definition (DTD). To introduce you to the
basic elements of a DTD, let’s begin with a simple example drawn from an XHTML DTD: