fox与ie的iframe 的document事件

<div style="width: 100px; height: 100px">main
    </div>
    <div style="width: 528px; height: 292px">iframe
    <iframe src="iframe.htm"></iframe>
</div>
<script>
document.onkeydown = function(){alert(100)};
var iframes = document.getElementsByTagName("iframe");
if ( isIE )
iframes[0].contentWindow.document.onkeydown = function(){alert(200)};
if ( isFox )
iframes[0].contentWindow.onkeydown = function(){alert(200)};
</script>