Eventually it becomes clear that any effort to discuss URLs or URIs is pointless.
Uniform Resource Identifiers are the strangest religion I've encountered on the Web. People use them in all kinds of largely incompatible ways, but somehow we're supposed to believe that since the URL part works and survives things like cache issues, these magical abstractions will solve all our identification problems.
Meanwhile, no one can give me a straight answer on how to identify a representation with a URI reference. The short answer, of course, is that you can't, though the fragment identifier part is amusingly representation-dependent and it seems like a representation must in some sense be a resource itself... but you'd better stay away from that hack of a DOS-like file extension or fall into sin.
The Web as a huge set of Platonic Forms would be hysterically funny if it weren't so throughly sad.