So moving forwards, can we stop accepting stuff that isn't HTTP/1.1 in our HTTP/1.1 server? Do we really want people to configure their server to speak "other"?
I'm starting to collect https://wiki.apache.org/httpd/Applications based on searching google for instances where users have toggled HttpProtocolOptions Unsafe, in response to specific application behavior. >From this list, we might decide to allow non-HTTP/1.1 input in specific cases, and perhaps have multiple grades of protocol correctness, as I first proposed. A trailing space, after the protocol designation in the request line is one example of a rather unambiguously safe incompatibility.
