DTDs and Document Structure

In the introduction, I used the phrase "DTD of some sort" because, in XML, technically there is no such thing as a single DTD. Although many XML developers think in terms of a single external DTD, the external DTD is not the only option for XML documents. To fully understand what a DTD is and how it is structured, you have to begin by learning about the role of the document type declaration.

The structure of an XML document is determined by the document type declaration, which appears in the prolog of an XML document. Within the document type declaration, markup declarations are provided that establish the grammar for a document or class of documents. The DTD is the actual grammar located within or referenced by the ...

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