A Document Type Definition (DTD) is a set of markup declarations that define a document type for an SGML-family markup language (SGML, XML, HTML).
A DTD uses a terse formal syntax that declares precisely which elements and references may appear where in the document of the particular type, and what the elements’ contents and attributes are. A DTD can also declare entities which may be used in the instance document.
XML uses a subset of SGML DTD.
As of 2009 newer XML namespace-aware schema languages (such as W3C XML Schema and ISO RELAX NG) have largely superseded DTDs. A namespace-aware version of DTDs is being developed as Part 9 of ISO DSDL. DTDs persist in applications which need special publishing characters such as the XML and HTML Character Entity References, which were derived from the larger sets defined as part of the ISO SGML standard effort.
Read more about Document Type Definition: Associating DTDs With Documents, Markup Declarations, XML DTDs and Schema Validation, XML DTD Schema Example
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