Its kind of hacking and its untested, but try
#myframe {
position:absolute;
top:40px;
left:40px;
width:100%;
}
then add "padding-bottom: 40px; padding-right: 40px" to the parent
EDIT
CSS Validated
div{
padding:40px;
position:absolute;
top:0px;
bottom:0px;
left:0px;
right:0px;
}
iframe{
height:100%;
width:100%;
}
HTML
<div><iframe></iframe></div>
source
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