iFrame is a html5 element like text box, they could not be overlapped by sprites.
I saw this post from a while back. I don't want to cast doubt on the OP, but is this true that you cannot change the z-order of an iframe? This seems to be confirmed when running the game both locally and from a server, but I was hoping there's some way to get around this issue. It's not the end of the world, but it's annoying.