下拉菜单被ul挡住的问题

<style>

/*导航*/

#webmenu {width:950px;margin:4px auto;z-index:999;}

#webmenu ul {padding-left:20px;height:27px;background: #000000;}

#webmenu li {float:left;font-size:14px;padding:6px 0;color:#FFFFFF;margin-right:5px;position:relative;}

#webmenu a:link,#webmenu a:visited {color:#FFFFFF;font-weight:bold;}



/*下拉菜单*/

#webmenu li div a:link,#webmenu li div a:visited{ display:block; text-decoration:none; color:#FFFFFF; width:100%; height:24px; line-height:24px; text-align:center;}

#webmenu li div a:hover{color:#FFFFFF; background:#000000; text-decoration:none;}

#webmenu li div{display:none; position:absolute; top:25px; left:0; width:100px;border: 1px solid #000000;border-left: 6px solid #000000; background: #000000;z-index:9999;}

</style>

<script>

function displaySubMenu(li){

var subMenu = li.getElementsByTagName("div")[0];

subMenu.style.display = "block";}



function hideSubMenu(li){

var subMenu = li.getElementsByTagName("div")[0];

subMenu.style.display = "none";}

</script>

<div id="webmenu">

	<ul>

<li> <a href="/" target="_top">首页</a></li><li onmouseover="displaySubMenu(this)" onmouseout="hideSubMenu(this)")>| <a href="" target="_top">网站运营</a><div>

<a href="" target="_top">站长新闻</a>

<a href="" target="_top">新手教程</a>

<a href="" target="_top">经验心得</a>

<a href="" target="_top">访谈</a>

<a href="" target="_top">推广策划</a>

<a href="" target="_top">搜索SEO</a>

</div>

</li></ul><ul>

<li onmouseover="displaySubMenu(this)" onmouseout="hideSubMenu(this)")> <a href="/class_11.html" target="_top">互联网</a><div>

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</div>

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</div>

</li></li>

</ul>

</div>