html to xhtml

I am picking down say a random web page (in html).

I want to transform some of the nodes. 
I convert the html string to xhtml (I can only do this on a file level
since
the memtomem don't seem to do the trick). Anyway I get what I think
is a tidy xhtml. But the set I get cannot be used againsted xsl, futhermore
the set I get does not seem to be tidy enough since the w3c test comes up
with tons of errormessages...

Any ideas?
H