<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8">
<title>折角效果</title>
<style type="text/css">
div {
width: 300px;
height: 200px;
background: #333;
position: relative;
}
div:before {
content: ‘‘;
width: 0px;
height: 0px;
position: absolute;
top: 0px;
right: 0px;
border-right: 0px solid #333;
border-bottom: 0px solid #768;
-webkit-transition: border .3s;
}
div:hover:before {
border-bottom-width: 30px;
border-right-width: 30px;
}
</style>
</head>
<body>
<div></div>
</body>
</html>
原文：http://www.cnblogs.com/djawh/p/4388426.html