XML - Document Type (Definition|Declaration) (DTD)


Document Type Definition (DTD) is a grammar (language) that defines the schema (ie data structure) of an XML document.



The syntax is a set of markup declarations that define a document type for SGML-family markup languages (SGML, XML, HTML).

DTDs were a precursor to XML schemas and have a similar function, although different capabilities.

A DTD can be declared:

  • inline inside an XML document,
  • or as an external reference.

The document type declaration can point to an external subset (a special kind of external entity) containing:

  • markup declarations,
  • or can contain the markup declarations directly in an internal subset,
  • or can do both.

The DTD for a document consists of both subsets taken together.

DTD are context-free grammars

Documentation / Reference

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