Validate (X)HTML in Python

What’s the best way to go about validating that a document follows some version of HTML (prefereably that I can specify)? I’d like to be able to know where the failures occur, as in a web-based validator, except in a native Python app.

Answer

XHTML is easy, use lxml.

from lxml import etree
from StringIO import StringIO
etree.parse(StringIO(html), etree.HTMLParser(recover=False))

HTML is harder, since there’s traditionally not been as much interest in validation among the HTML crowd (run StackOverflow itself through a validator, yikes). The easiest solution would be to execute external applications such as nsgmls or OpenJade, and then parse their output.