DIV 掩住select input

<div id= "menu " style= "position:absolute; visibility:hidden; top:20px; left:20px; width:100px; height:200px; background-color:#6699cc; ">
<table>
<tr> <td> item 1 </td> </tr>
<tr> <td> item 2 </td> </tr>
<tr> <td> item 3 </td> </tr>
<tr> <td> item 4 </td> </tr>
<tr> <td> item 5 </td> </tr>
</table>
<iframe src= "javascript:false " style= "position:absolute; visibility:inherit; top:0px; left:0px; width:100px; height:200px; z-index:-1; filter= 'progid:DXImageTransform.Microsoft.Alpha(style=0,opacity=0) '; "> </iframe>
</div>

<a href= "# " onclick= "document.getElementById( 'menu ').style.visibility= 'visible ' "> menu </a>

<form>
<select> <option> A form selection list </option> </select>
</form>