It's not valid for a compiler to make demons fly out of your nose, since compilers are constrained by the capabilities of your hardware and the laws of physics.
Well, if a compiler were to do so it would not be in violation of the standard. Whether or not such a thing is physically possible is an implementation detail outside the scope of the standard.
The C standard isn't the only thing compilers have to comply with.
HTTP doesn't specify what happens if the request method is XYZZY. An HTTP server that confirms to the HTTP specification (but not the laws of physics) could make demons fly out of your nose when receiving a request with a method of XYZZY. But nobody complains about that.
