Post a link to the page. The css for only one element is just part of the story.
Is the iframe inside mydiv? and you want mydiv in a fixed position 0,0 top left? and you want mydiv to will entire window? ... but iframe contents may not be large enough to so that.
Anyway, post a link and describe again what you are attempting, then we will be better able to visualize and assist.
Eye for Video