我设置@media (max-width: 820px)在pc页面小于820也无效在移动端才有效

来源：4-4 自由编程

WebSaber

2019-10-14 21:29:50

<!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>
        *{
            list-style: none;
            padding: 0;
            margin: 0;
            box-sizing: border-box;
        }
        header{
            height: 100px;
            width: 100%;
            border-bottom: 1px solid gray;
            display: flex;
            align-items: center;
            position: relative;
            overflow: hidden;
        }
        header ul{
            display: flex;
            position: absolute;
            left: 0;
        }
        header ul li{
            margin-left: 50px;
        }
        header img{
            margin: 0 auto;
            width: auto;
            height: 100px;
        }
        header div{
            height: 200px;
            width: 200px;
            background: red;
            border-radius: 50%;
            color: #fff;
            position: absolute;
            left: -100px;
            bottom: -100px;
            display: none;
        }
        header div b{
            height: 0;
            width: 40px;
            border: 1px #fff solid;
            position: absolute;
            right: 50px;
            top: 50px;
        }
        header div b:last-child{
            top: 70px;
        }
        .none{
            width: 100%;
            height: 0px;
            background: rgb(201, 200, 200);
            overflow: hidden;
            position: relative;
            top: -1px;
            transition: height 0.5s;
        }
        .text-none{
            height: 151px;
            top: 0px;
        }
        .none ul li{
            width: 100%;
            line-height: 50px;
            border-bottom: 1px solid gray;
            padding-left: 30px;
        }
        @media (max-width: 820px){
            header ul{
                display: none;
            }
            header div{
                display: block;
            }
        }
    </style>
</head>
<body>
    <header>
        <div>
            <b></b><b></b>
        </div>
        <ul>
            <li>首页</li>
            <li>关于</li>
            <li>商业合作</li>
        </ul>
        <img src="https://www.imooc.com/static/img/index/logo.png" alt="logo">
    </header>
    <div class="none" id="none">
        <ul>
            <li>首页</li>
            <li>关于</li>
            <li>商业合作</li>
        </ul>
    </div>
    <script>
        var div=document.getElementsByTagName("div")[0];
        var none=document.getElementById("none");
        div.onclick=function(){
            if(none.classList.contains("text-none")){
                none.classList.remove("text-none");
            }else{
                none.classList.add("text-none");
            }
        }
    </script>
</body>
</html>

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