Document type definition


Document type definition (DTD) is a set of markup declarations that define a document type for an SGML-family markup languages (SGML, XML, HTML).

Document type definition defines the legal building blocks of an XML document which includes

  • the document structure
  • a list of legal XML elements and attributes.
A DTD can be declared inline inside an XML document, or as an external reference.

XML Document type definition uses a subset of SGML Document type definition.

As of 2009, newer XML namespace-aware schema languages (such as W3C XML Schema Definition Language (XSD) and ISO RELAX NG) have largely superseded Document type definitions.

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