CSS：二级菜单

html:

<!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>
    <link rel="stylesheet" href="css/reset.css">
    <link rel="stylesheet" href="css/二级菜单.css">
</head>
<body>
    <header class="header">
        <ul class="topnav clearfix">
            <li><a href="">Lorem.</a></li>
            <li><a href="">Repellat!</a></li>
            <li><a href="">Necessitatibus?</a></li>
            <li>
                <a href="">客户服务</a>
                <div class="submenu">
                   
                    <ul>
                        <li>Lorem.</li>
                        <li>Dolore.</li>
                        <li>Sed.</li>
                        <li>Excepturi.</li>
                        <li>Inventore.</li>
                        <li>Expedita.</li>
                        <li>Sunt.</li>
                        <li>At.</li>
                        <li>Nisi.</li>
                        <li>Tempore!</li>
                    </ul>
                </div>
            </li>
            <li><a href="">At.</a></li>
        </ul>
    </header>

    <div style=" background:lightblue;">
    </div>
</body>
</html>

css:

.clearfix::after {
    content: "";
    display: block;
    clear: both;
}

.header {
    background: #e3e4e5;
    color: #999;
    height: 40px;
    line-height: 40px;
    position: fixed;
    width: 100%;
    left: 0;
    top: 0;
}

.header .topnav>li {
    float: left;
    margin: 0 20px;
    width: 150px;
    text-align: center;
    height: 40px;
    box-sizing: border-box;
    position: relative;
}

.header .topnav>li:hover{
    background: #fff;
    border: 2px solid #ccc;
    border-bottom: none;
    line-height: 36px;
    background: #fff;
}

.header .topnav>li .submenu{
    text-align: left;
    line-height: 1.5;
    width: 300px;
    /* 不生成盒子 */
    display: none; 
    border: 2px solid #ccc;
    box-sizing: border-box;
    position: absolute;
    right: -2px;
    background: #fff;
}

.header .topnav>li:hover .submenu{
    display: block;
    padding: 10px;
}

.header .topnav>li:hover::after{
    content: "";
    position: absolute;
    width: 100%;
    height: 5px;
    background: red;
    bottom: 0;
    left: 0;
    background: #fff;
}

CSS：二级菜单

CSS：二级菜单

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