为什么我这样达不到清除浮动效果呢？

来源：5-4 编程练习

tttjh_

2021-02-19 00:13:08

相关代码：

<!DOCTYPE html>
<html>
<head>
    <meta charset="UTF-8">
    <title>Document</title>
    <style>
        .per{
            width: 300px;
            height: auto;
            border:2px solid #aaa;
        }
        .li{
            width: 100px;
            height: 100px;
            background: red;
            margin: 5px;
            float: left;
        }
        #clear :: after{
            content: "";
            visibility: hidden;
            height: 0px;
            display: block;
            clear: both;
        }
        /*此处写代码*/
    </style>
</head>
<body>
    <div class="per">
        <div class="li"></div>
        <div class="li"></div>
        <!-- 此处写代码 -->
        <div id="clear"></div>
    </div>
</body>
</html>