<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title>margin</title>
<style>
*{ margin: 0px; }
.father { width: 400px; height: 400px; background-color: #999;/* overflow: hidden;*/}
.son { width: 200px; height: 200px; background-color: red; margin-top: 20px;}
</style>
</head>
<body>
<div class="father">
<div class="son"></div>
</div>
</body>
</html>
效果：
解决办法：给发元素加个overflow: hidden;