!!! Overview
[{$pagename}] ([DTD]) is a set of markup declarations that define a document type for an [SGML]-family markup languages ([SGML], [XML], [HTML]).

[{$pagename}] 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] [{$pagename}] uses a subset of [SGML] [{$pagename}].

As of [2009|Year 2009], newer [XML namespace]-aware schema languages (such as [W3C] [XML Schema Definition Language] ([XSD]) and ISO RELAX NG) have largely superseded [{$pagename}]s. 

!! More Information
There might be more information for this subject on one of the following:
[{ReferringPagesPlugin before='*' after='\n' }]
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* [#1] - [Document_type_definition|Wikipedia:Document_type_definition|target='_blank'] - based on information obtained 2018-01-08-