As for the HTTP/1.0 protocol: I'd say:
An HTTP server must specify the content type of the response
data. There is no default.
This is more or less what I said, except that I allowed for Content-Type to
be ommitted if there is no response data.
In practice HTTP/0.9 result streams are untyped at the HTTP level. Client
pick their own types. Attempting to retroactively change reality by fiat is
silly. Now, changing reality by citroen is much more sensible.