div等浮层在IE6下被下拉框遮挡的解决方法

<style type="text/css">
dl #iframe1{display:none;_display:block;position:absolute;:0;left:0px;z-index:-1;filter:mask();width:110px;height:420px;}
</style>

<li id="dictionary" runat ="server" ><a href="#">维护 </a>
<dl >
<dd>
<a href="dictAll.aspx?TableType=0" target="_self" title="The">资产类型</a>
</dd>
<dd>
<a href="dictAll.aspx?TableType=13" target="_self" title="Styling">服务级别</a>
</dd>
<iframe id="iframe1"></iframe>
</dl>
</li>