I am fine with locking down cross-origin iframes, but one major annoyance is the inability to detect whether the iframe has a scrollbar and if so, adjust the size until it doesn't have one. I really wish that there was a "height: auto" property for iframes that automatically adjusted the height so that it didn't need a scrollbar.
Once upon a time the specs planned a <iframe seamless>, but unfortunately it was never widely implemented. Probably because it also tried to do things like letting the outer CSS leak into the seamless frame.
