The declaration is the very first thing in your document, before the tag. This tag tells the browser which HTML or XHTML specification the document uses.

Note: The tag does not have an end tag!

HTML

HTML 4.01 specifies three document types: Strict, Transitional, and Frameset.

HTML Strict DTD

Use this when you want clean markup, free of presentational clutter. Use this together with Cascading Style Sheets (CSS):

HTML Transitional DTD

The Transitional DTD includes presentation attributes and elements that W3C expects to move to a style sheet. Use this when you need to use HTML's presentational features because your readers don't have browsers that support Cascading Style Sheets (CSS):

Frameset DTD

The Frameset DTD should be used for documents with frames. The Frameset DTD is equal to the Transitional DTD except for the frameset element replaces the body element:

XHTML

XHTML 1.0 specifies three XML document types: Strict, Transitional, and Frameset.

XHTML Strict DTD

Use this DTD when you want clean markup, free of presentational clutter. Use this together with Cascading Style Sheets (CSS):

XHTML Transitional DTD

Use this DTD when you need to use XHTML's presentational features because your readers don't have browsers that support Cascading Style Sheets (CSS):

XHTML Frameset DTD

Use this DTD when you want to use frames!

To check that you have written a valid XHTML document with a correct DTD, you can link your XHTML page to an XHTML validator.