I've also tried with IE=9 and it doesn't work. The iFrame always render using the parent's document mode.
The problem is that my iFrame is using "-ms-transform" CSS property to rotate an image, which only works with IE9 (and that's the only IE version we support). That function doesn't work when the parent is in Quirks Mode, but works when the parent
is forced to use IE9 document mode (hence the iFrame also renders as IE9). The parent is an old site so we cannot set the parent to use IE9 document mode, because it renders broken.