Document type definition (DTD
) is a set of markup declarations that define a document type for an SGML
-family markup languages (SGML
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.
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XML namespace-aware schema languages (such as W3C XML Schema Definition Language (XSD) and ISO RELAX NG) have largely superseded Document type definitions.
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