<!DOCTYPE HTML>
<html>
    <head>
        <script>
            function test()
            {
                document.getElementById("target").style.height = "100px";
            }
        </script>
    </head>
    <body>
        <div style="height: 50px; background: red; width: 100px;" id="target">
            <div style="height: 100%">
                <div></div>
                <iframe style="vertical-align: bottom; width: 100%; height:100%; background: green; border: 0;"></iframe>
            </div>
        </div>
    </body>
    <script>
        document.body.offsetTop;
        test();
    </script>
</html>