本贴代码实现的是“用em实现倒三角的提示效果”。
仅供新手参考学习，欢迎大家提出更多的实现方法。
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=gb2312" />
<title>em实现倒三角的提示效果</title>
</head>
<style>
#menu {width:500px;padding:0;margin:40px auto;list-style-type:none;display:<span href="tag.php?name=tab" onclick="tagshow(event)" class="t_tag">tab</span>le;}
#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:6px 6px 0 6px; position:absolute; left:50%; top:100%;margin-left:-6px;}
</style>
<body>
<ul id="menu">
<li><a href="#"><span>M One</span><em></em></a></li>
<li><a href="#"><span>M Two</span><em></em></a></li>
<li><a href="#"><span>M Three</span><em></em></a></li>
<li><a href="#"><span>M Four</span><em></em></a></li>
</ul>
</body>
</html>