<html>
<head>
<title>带倒三角标记的WEB标准菜单</title>
<style>
<!--
#menu {width:500px;padding:0;margin:40px auto;list-style-type:none;}
#menu li {width:100px; float:left; line-height:30px}
#menu a {position:relative;display:block; text-decoration:none; background:#cccccc;width:100px; }
#menu a span {display:block; font-weight:bold;color:#000; border-style:solid;border-width:0px 2px 2px 0px; border-color:#fff #fff #06a #fff;text-align:center; }#menu a em {display:none;}
#menu a:hover {background:#FF0000;}
#menu a:hover span {color:#fff; }
#menu a:hover em {display:block; overflow:hidden; border:6px solid #06a; border-color:#06a #fff; border-width:10px 6px 0 6px;top:100%;margin-left:-6px; position:absolute; left:50%; }
-->
</style>
</head>
<body>
<ul id="menu">
<li><a href="#"><span>脚本之家</span><em></em></a></li>
<li><a href="#"><span>网页特效</span><em></em></a></li>
<li><a href="#"><span>示例菜单</span><em></em></a></li>
<li><a href="#"><span>网站导航</span><em></em></a></li>
</ul>
</body>
</html>
hover伪类而已啊，http://www.w3school.com.cn/cssref/selector_hover.asp
#menu a:hover em {/*鼠标移动到id为menu下的a元素时，a下的em元素显示出来*/
display: block;
overflow: hidden;
border: 6px solid #06a;
border-color: #06a #fff;
border-width: 10px 6px 0 6px;
top: 100%;
margin-left: -6px;
position: absolute;
left: 50%;
}