<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8" />
<title>正方形</title>
<style>
.wrap{width:40%;background:red;}
.box{padding-top:100%;overflow:hidden;}
.child{margin-top:-100%;}
</style>
</head>
<body>
<div class="wrap">
<div class="box">
<div class="child">這個是正方形</div>
</div>
</div>
</body>
</html>
