<head>
<meta charset="UTF-8">
<title></title>
<style>
#parent {
border: 1px solid blueviolet;
}
#childone {
float: left;
width: 200px;
height: 400px;
background-color: #AFEEEE;
}
#childtwo {
float: left;
width: 200px;
height: 200px;
background-color: #FFB6C1;
}
#parent::after {
content: "";
display: block;
/*height: 0;*/
clear: both;
/*visibility: hidden;*/
}
</style>
</head>
<body>
<div id="parent">
<div id="childone"></div>
<div id="childtwo"></div>
</div>
</body>
after伪元素必须配合content使用
clear：both只在块级元素上生效