Actually, that's a pretty good point. The thing is that the URL itself refers to some conceptual resource, and the response is ideally a representation of that resource, potentially one of multiple. If you take the same source image and encode it multiple ways, although the two resulting images are different from eachother, they are representations of the same underlying image/resource. But if you were to provide a different image, or alter the image in other ways, I think this would be pretty tricky actually, even if the modification was something trivial. You can imagine a simple use case like a WebP image with the text "Your browser supports WebP" and a PNG image with the text "Your browser does not support WebP." The point is that content negotiation being used to present different logical resources that are not necessarily interchangable representations of the same data feels tricky. I think that is still compatible with the fact that it's all within specifications.