• DTD specifies the syntax of a web page in SGML.
• DTD is used by SGML applications, such as HTML, to specify rules that apply to the markup of documents of a particular type, including a set of element and entity declarations.
• XHTML is specified in an SGML document type definition or 'DTD'.
• An XHTML DTD describes in precise, computer-readable language, the allowed syntax and grammar of XHTML markup.

There are currently 3 XHTML document types:

• STRICT
• TRANSITIONAL
• FRAMESET