xhtml can't handle the unclosed <p> tag the way HTML could. If you are sticking with HTML 4.0 most CSS browsers who dislike the assumed closure of the P tag can be spoofed with the very evil xmlish <p/> but please don't tell anyone _I_ told you that. =)

As xhtml becomes more prevalent, lightweight checkers will become more popular and handy.

--
$you = new YOU;
honk() if $you->love(perl)