js访问iframe中的DOM

<iframe name="feedbackform" ></iframe>
<script type="text/javascript">
var temp_css;
temp_css="<style type=\"text/css\">";
temp_css+="body{padding:10px 0;margin:0;font-size:12px;}";
temp_css+="</style>";
window.onload=function(){
 var obj=window.frames["feedbackform"];
 obj.document.body.innerHTML+=temp_css;
}
</script>