JavaScript is not a real solution to this problem, since it takes the control away from the client and condemns web developers to maintaining those horrible hac

JavaScript is not a real solution to this problem, since it takes the control away from the client and condemns web developers to maintaining those horrible hacks forever as clients with unforeseen capabilities or expectations emerge. We need something that allows developers to add support for 2x images without having the server make any assumptions about what the client wants. If the client wants 1x images for whatever reason, it doesn't matter what their screen resolution or pixel scaling factor is, or whatever your script is using to determine support for 2x image display.