<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<title></title>
<style>
.longen{
width:300px;
height:100px;
border:5px solid #09F;
position:relative;
background-color:#FFF;
}
.longen:before,.longen:after{
content:"";display:block;
border-width:20px;
position:absolute; bottom:-40px;
left:100px;
border-style:solid dashed dashed;
border-color:#09F transparent transparent;
font-size:0;
line-height:0;
}
.longen:after{
bottom:-33px;
border-color:#FFF transparent transparent;
}
</style>
</head>
<body>
<div >
css3气泡框
</div>
</body>
</html>
