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DTD(Document Type Definition) A language that describes the contents of an XML or SGML document. Written in their unique syntax (not XML or SGML), the DTD definitions may be embedded within the document or in a separate file. XML schemas, such as W3C XML Schema and RELAX NG, are also used to define the content of an XML document. See XML schema.
DTD in HTML/XHTML Pages
The first line of code in a Web page is typically a DOCTYPE, which refers to a DTD that defines the markup rules for rendering the Web page (see DOCTYPE).