XHTML 1.0

Three flavors -- three DTDs.

XHTML 1.0 Strict
For really clean structural mark-up, free of any markup associated with layout.
XHTML 1.0 Transitional
take advantage of XHTML features including style sheets but nonetheless to make small adjustments to your markup for the benefit of those viewing your pages with older browsers which can't understand style sheets. These include using the body element with bgcolor, text and link attributes.
XHTML 1.0 Frameset
When using Frames to partition the browser window into two or more frames.