菜单项无法使用IE呈现

Question

如果我在Mozilla Firefox下面呈现HTML代码，那就好了。如果在IE（9.0.8112）中渲染相同，则不显示我的菜单项。如何解决这个问题（默认情况下是Windows 7）？

CSS

ul#nav {margin: 0 0 0 10px;}
ul.drop a { display:block; color: #fff; font-family: Verdana; font-size: 13px;     text-decoration: none;}
ul.drop, ul.drop li, ul.drop ul { list-style: none; margin: 0; padding: 0; border: 1px solid #fff; background: #555; color: #fff;}
ul.drop { position: relative; z-index: 597; float: left; }
ul.drop li { float: left; line-height: 1.3em; vertical-align: middle; zoom: 1; padding: 5px 40px; }
ul.drop li.hover, ul.drop li:hover { position: relative; z-index: 599; cursor: default; background: #1e7c9a; }
ul.drop ul { visibility: hidden; position: absolute; top: 100%; left: 0; z-index: 598; width: 195px; background: #555; border: 1px solid #fff; }
ul.drop ul li { float: none; }
ul.drop ul ul { top: -2px; left: 100%; }
ul.drop li:hover > ul { visibility: visible }

HTML

<ul id="nav" class="drop">

   <li>Projects
    <ul>
      <li><a href="#">All Projects</a></li>
      <li><a href="#">Proj Details</a></li>
    </ul>
  </li>

  <li>Dailies
    <ul>
      <li><a href="/addTask/fillTask/">Add Task</a></li>
      <li><a href="#">Add Status</a></li>
    </ul>
  </li>

  <li>Add Data
    <ul>
      <li><a href="/addProjHtml/">Add Proj</a></li>
      <li><a href="#">Add Assets</a></li>
      <li><a href="#">Add Scenes</a></li>
    </ul>
  </li>