OT: HTML/XHTML syntax checking tools?

[George Sanderson]

What HTML/XHTML syntax tools are recommended?  I was hoping that the tool
would run on the browser (client) side.  I prefer a PC version, but hey, a
good ap is best.-)

For example, I am working on a mod_perl module, and I made a HTML typo
(TEST instead or TEXT), (belzlare slip).  Perhaps I or someone else has
made other mistakes.  Since the browser ignored the TEST tag, and the
default was reasonable, it looked OK.  Now I think I should be checking
with something other than my eye.