1.父元素设置overflow: hidden
2.父原始设置border
3.在竖直方向，善于使用padding，而不是margin
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<meta http-equiv="X-UA-Compatible" content="ie=edge">
<title>Document</title>
<style>
* {
margin: 0;
padding: 0
}
.parent{
background: green;
width: 200px;
height: 200px;
/* overflow: hidden; */
/* border: 1px solid red; */
padding: 50px 0px;
}
.children{
width: 100px;
height: 100px;
/* margin-top: 50px; */
background: red;
}
</style>
</head>
<body>
<div class="parent">
<div class="children"></div>
</div>
</body>
</html>
