"HTML 4 extends HTML with mechanisms for style sheets, scripting, frames, embedding objects, improved support for right to left and mixed direction text, richer tables, and enhancements to forms, offering improved accessibility for people with disabilities.
HTML 4.01 is a revision of HTML 4.0 that corrects errors and makes some changes since the previous revision." 
The HTML 4.01 recommendation comprises three Document Type Definitions (DTD):
The Default DTD is HTML 4.01 Strict DTD. It "excludes the presentation attributes and elements that W3C expects to phase out as support for style sheets matures." 
The HTML 4.01 Transitional DTD includes presentation attributes and elements that W3C expects to phase out as support for style sheets matures.
The HTML 4.01 Frameset DTD should be used for documents with frames. "This DTD is identical to the HTML 4.01 Transitional DTD except for the content model of the 'HTML' element: in frameset documents, the 'FRAMESET' element replaces the 'BODY' element." 
In order to achieve a strict separation between structure and presentation we recommend to take only the Strict DTD of HTML 4.01 into consideration. The Transitional DTD and Frameset DTD should not be used.
Copyright © 2001-2003 by Rainer Hillebrand and Thomas Wierlemann