HTML and CSS Reference
With the transitional XHTML DTD, you can ease into the XML family while
staying current with the browser industry.
The third DTD is for frames. It is identical to the transitional DTD in all
other respects; the only difference is the replacement of the document
body with appropriate frame elements. You might think that, for com-
pleteness's sake, there would be strict and transitional frame DTDs, but
the W3C decided that if you use frames, you might as well use all the
deprecated elements as well.