<style>
.b_main{position:absolute}
.out{
position:absolute;
top:7px;
border-color:transparent #999999 transparent transparent;
border-style:dashed dashed solid dashed;
border-8px;
}
.in{
position:absolute;
border-color:transparent #ffffff transparent transparent;
border-style:dashed dashed solid dashed;
border-8px;
top:-8px;
left:-6px;
}
.border{
position:relative;
border:1px solid #999999;
padding:10px;
left:16px;
border-radius:5px;
}
</style>
</head>
<body>
<div class="b_main">
<div class="border">效果实现^_^</div>
<div class="out">
<div class="in"></div>
</div>
</div>
</body>
http://www.jb51.net/css/143000.html
http://www.oschina.net/code/snippet_125027_3637
http://www.yuiblog.com/blog/2010/11/22/css-quick-tip-css-arrows-and-shapes-without-markup/