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It sounds like this is a case where the docs should mention an external library; perhaps something like changing the intro of http://docs.python.org/dev/library/html.parser.html from:
"""
19.2. html.parser — Simple HTML and XHTML parser
Source code: Lib/html/parser.py
This module defines a class HTMLParser which serves as the basis for parsing text files formatted in HTML (HyperText Mark-up Language) and XHTML.
"""
to:
"""
19.2. html.parser — Simple HTML and XHTML parser
Source code: Lib/html/parser.py
This module defines a class HTMLParser which serves as the basis for parsing text files formatted in HTML (HyperText Mark-up Language) and XHTML. 
Note that mainstream web browsers also attempt to repair invalid markup; the algorithms for this can be quite complex, and are evolving too quickly for the Python release cycle. Applications handling arbitrary web pages should consider using 3rd-party modules. The python version of html5lib ( http://code.google.com/p/html5lib/ ) is being developed in parallel with the HTML standard itself, and serves as a reference implementation.
"""