Whilst you can't say ‘height: 100% minus some pixels’ in CSS, you can make the iframe 100% high, then push its top down using padding. Then you can take advantage of the CSS3 box-sizing property to make the padding get subtracted from the height.
This:
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
<html><head>
<title>test</title>
<style type="text/css">
html, body { margin: 0; padding: 0; height: 100%; }
#bar { height: 32px; background: red; }
iframe {
position: absolute;
top: 0; left: 0; width: 100%; height: 100%;
border: none; padding-top: 32px;
box-sizing: border-box; -moz-box-sizing: border-box; -webkit-box-sizing: border-box;
}
</style>
</head><body>
<iframe src="http://www.google.com/"></iframe>
<div id="bar">foo</div>
<body></html>
Works on IE8, Moz, Op, Saf, Chrome. You'd have to carry on using a JavaScript fallback to make the extra scrollbar disappear for browsers that don't support box-sizing though (in particular IE up to 7).
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