I can see that using "Accept" can be considered a more elegant solution, I don't see how adding a parameter will break HTTP. There is nothing in the HTTP standard that says what an URL should or should not return. The Accept-header is just an additional source of information.
Not actually, the Accept header isn't a mere suggestion. If the server chooses not to fulfill it, the correct response is 406 Not Acceptable. This is why browsers send something like this in their Accept header:
