How to parse malformed HTML in python, using standard libraries

Parsing HTML reliably is a relatively modern development (weird though that may seem). As a result there is definitely nothing in the standard library. HTMLParser may appear to be a way to handle HTML, but it's not -- it fails on lots of very common HTML, and though you can work around those failures there will always be another case you haven't thought of (if you actually succeed at handling every failure you'll have basically recreated BeautifulSoup).

There are really only 3 reasonable ways to parse HTML (as it is found on the web): lxml.html, BeautifulSoup, and html5lib. lxml is the fastest by far, but can be a bit tricky to install (and impossible in an environment like App Engine). html5lib is based on how HTML 5 specifies parsing; though similar in practice to the other two, it is perhaps more "correct" in how it parses broken HTML (they all parse pretty-good HTML the same). They all do a respectable job at parsing broken HTML. BeautifulSoup can be convenient though I find its API unnecessarily quirky.

Take the source code of BeautifulSoup and copy it into your script ;-) I'm only sort of kidding... anything you could write that would do the job would more or less be duplicating the functionality that already exists in libraries like that.

If that's really not going to work, I have to ask, why is it so important that you only use standard library components?

Your choices are to change your requirements or to duplicate all of the work done by the developers of third party modules.

Beautiful soup consists of a single python file with about 2000 lines of code, if that is too big of a dependency, then go ahead and write your own, it won't work as well and probably won't be a whole lot smaller.