DTD. Document Type Declaration and Definittion are used to declare and help a web browser dertermind the type of Html document the Web Author wrote. HTML 4.01 specifies Three Types of Doctype Definitions. Each DTD vary in the elements they support, so authors must include one of the following document type declarations in their Html documents (Webpages).
A valid HTML/XHTML document declares what version of HTML/XHTML is used in the document. The document type declaration names the document type definition (DTD) in use for the document.
HTML 4.01 Has Three Types of Doctype Definitions
HTML 4.01 specifies three DTDs, so authors must include one of the following document type declarations in their documents. The DTDs vary in the elements they support.
The Global Structure of an HTML Document - W3C DOCTYPE Declaration in Documents - Complete Listing of DTDs - W3C Fixing your site with the right DOCTYPE
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