I think this is a bit of an ambitious request, but hopefully it's something that we can achieve.

The HTTP spec contains a lot of "gotchas" and unless a contributor knows the HTTP spec inside-out (which is almost impossible), we're bound to make a change at some point that violates the spec. A good example of this is #3080, where a previous fix to an issue that allowed users to provide an invalid content-length (which is against the spec) seems to have broken HEAD requests, where the content-length should match the length of the content as if the request is a GET request.

In order to guard against this, we need an HTTP Spec conformance test suite that runs during CI. A potentially good starting point is this repo, but there are probably other / better tools for it (haven't looked too extensively). I'm sure that adding such a suite will very likely unearth violations, so we should probably do it in 2-3 steps:

1. Add a CI workflow that tests a server for HTTP spec conformance.
2. Fix any failing checks that are required by the spec.
3. Fix as many recommendations as possible.

@jdegoes @987Nabil what are your thoughts on having this? Is this something that's too ambitious or do you think it might be doable?