div在IE7中消失

<div id=\"top_menu\">
    <ul id=\"dropmenu\">
        <li>menu item1</li>
        <li>menu item2</li>
        ...
     </ul>
    <div class=\"lang\">content</div>
</div><!-- end top menu -->

  #top_menu                     {width:900px;font-size:13px; }
    #top_menu ul#dropmenu           {width:630px; height:28px; margin:0px; padding:0px; list-style:none; float:left; }
    #top_menu ul#dropmenu li        {float:left;display:block;}
    .clearfix                       {display: inline-block;}  /* for IE/Mac */

    #top_menu .lang { width: 120px; color:#fff; margin:4px 10px 0 0; float: right; }
    #top_menu .lang a{ color:#ff8601; }
    #top_menu .lang a:hover{ color:#fff; }