避免margin穿透问题，解决方法之4margin自身元素bfc化无法解决啊
<!DOCTYPE html>
<html>
<head>
<meta charset="UTF-8">
<title>Document</title>
<style>
* {
margin: 0;
padding: 0;
}
body {
background-color: grey;
}
.parent {
width: 400px;
height: 200px;
background-color: black;
overflow: hidden;
}
.son {
width: 200px;
height: 100px;
background-color: green;
margin-top: 20px;
}
</style>
</head>
<body>
<div>
<div></div>
</div>
</body>
</html><!--这段代码在chrome 火狐 edge都测试过可以避免margin穿透-->