You could easily use display:table to vertical-align content and text-align:center to horizontal align your iframe. http://jsfiddle.net/EnmD6/7/
html {
display:table;
height:100%;
width:100%;
}
body {
display:table-cell;
vertical-align:middle;
}
#top-element {
position:absolute;
top:0;
left:0;
background:orange;
width:100%;
}
#iframe-wrapper {
text-align:center;
}
version with table-row http://jsfiddle.net/EnmD6/9/
html {
height:100%;
width:100%;
}
body {
display:table;
height:100%;
width:100%;
margin:0;
}
#top-element {
display:table-row;
background:orange;
width:100%;
}
#iframe-wrapper {
display:table-cell;
height:100%;
vertical-align:middle;
text-align:center;
}