Standards are just commonly accepted behaviour that somebody chose to write down somewhere. There are a great number of commonly accepted behaviours that nobody's ever bothered to encode into a formal standard, but where failure to follow the accepted practice will result in widespread breakage. There are also a great many "standards" that you would be a fool to follow to the letter. In the OP case, the only thing that will break is people trying to visit their site, who will presumably simply press the back button on their browser and go about their day. They can decide for themselves if that is an acceptable casualty. But it isn't definitionally acceptable because no standard says it isn't (nor would is suddenly become unacceptable because a standard said it was...)