[eadmund]

> I must make fewer assumptions about the response, for example that they are well-formed or even match the requested content-type.

I think that you should always assume that an HTTP response is a well-formed HTTP response (otherwise you can't even trust that the 404 itself is correct); and you should never assume that the received MIME type is the same as the ones you indicated you accepted; you should always check the Content-type header.