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<title> new document </title>
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a span { display:inline-block; border:0 solid #fff; border-width:5px 5px 0; border-color:#090 #fff;width:0; overflow:hidden;height:0; }
a:hover{ border:none;}
a:hover span { border-width:0 5px 6px; }
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<p>
<a href="">悬浮-下拉菜单<span></span></a>
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原文：http://my.oschina.net/glelaine/blog/288312