> I'm surprised there doesn't seem to be any widely-used and easily available HTTP conformance checker - unlike the well-known HTML validators.
There is one called Co-Advisor [1] that can be used to test web proxies. It is commercial and pretty expensive, but the online version might be free for open source projects. Squid and Apache Traffic Server are tested with it [2][3]. There was a USENIX talk that showed some Co-Advisor results [4]