<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title>Title</title>
<style>
.aaa{
width:200px;
height:200px;
position:relative;
background: red;
overflow: hidden;
}
.aaa .bbb{
width:100px;
height:100px;
position: absolute;
background: green;
top:-50px;
right:-50px;
transform: rotate(45deg);
}
.aaa .bbb span{
position: absolute;
bottom:0;
display: block;
width:100px;
text-align: center;
}
</style>
</head>
<body>
<div class="aaa">
<div class="bbb">
<span>四年级</span>
</div>
</div>
</body>
</html>
原文：http://www.cnblogs.com/qiancheng123/p/6346404.html