<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title>Document</title>
<style type="text/css">
#container {
width: 400px;
}
#target {
width: 100%;
padding-bottom: 100%;
background-color: #f00;
}
</style>
</head>
<body>
<div id="container">
<div id="target">
</div>
</div>
</body>
</html>
效果：