Using DTDs
The DTD validation guarantees that the source document complies with the validity constraints defined in a separate file—the DTD. A DTD file uses a formal grammar to describe both the structure and the syntax of XML documents. XML authors use DTDs to narrow the set of tags and attributes allowed in their documents. Validating against a DTD ensures that processed documents conform to the specified structure. From a language perspective, a DTD defines a newer and stricter XML-based syntax and a new tagged language tailor-made for a related group of documents.
Historically speaking, the DTD was the first tool capable of defining the structure of a document. The DTD standard was developed a few decades ago to work side by side with SGML—a ...
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