The attached patch include these changes:
* strict=False default
* strict arg deprecated in the doc
* strict=True deprecated (raises a warning)
* HTMLParseError deprecated in the doc
* some calls to HTMLParser.error converted to asserts [0]

Regarding
* HTMLParser.error deprecated (raises a warning)
I'm not sure anymore that's a good idea.  The method is not documented, so in theory it could be removed without deprecation warnings, but that might break things if someone is using it.

[0] I made a mistake in my first message: some of the calls should actually be converted to assert, the others will stay as long as the strict mode exists (i.e. they will be removed in 3.5)