DTD:  Document Type Definition [http://www.w3.org/TR/REC-xml#dt-doctype]

Adjunct to [XML] and [SGML]. markup declarations that provide a grammar for a class of documents. Defines how tags can be nested, follow each other, etc. Essentially describes the contents of each valid tags as a regular expression. Beyond that it defines the attributes per tag, and their type.

Simple example:
 <!ELEMENT database (record+) >
 <!ELEMENT record (firstname,lastname,phone+)>
 <!ATTLIST record
   created NMTOKEN #IMPLIED
 >
 <!ELEMENT firstname (PCDATA)>
 <!ELEMENT lastname  (PCDATA)>
 <!ELEMENT phone     (PCDATA)>

[tDom] with its extension [tnc] can validate XML documents by a given DTD.
[LV] 2007 Oct 09 - it isn't clear how tdom can validate xml documents by a given DTD.

As the language of DTDs differs from XML, it is considered old-fashioned, and several replacements (which follow XML syntax) have been proposed: XML Schema, ...
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MAKR: There is fun project called [A little XML Schema validator] ...


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