The standard answer to this is that there's too many variants. I think it should be a thing anyway. If Blink, Webkit and Firefox have compatible implementation

The standard answer to this is that there's too many variants. I think it should be a thing anyway. If Blink, Webkit and Firefox have compatible implementations there can be some sort of de facto standard and it will be fine for a number of actually useful things, even if it's not perfectly defined to pocket protector standards.