<style>    ul{        list-style: none;        margin: 0;        padding: 0;    }    ul li a{        display: block;        text-decoration: none;        width: 100px;        height: 50px;        text-align: center;        line-height: 50px;        color: white;        background-color: #2f3e45;    }    .drop-down{        width: 100px;        height: 50px;    }    .drop-down-content{        opacity: 0;        height: 0;        overflow: hidden;        transition: all 1s ease;    }    p{        font-size: 20px;        margin: 0;    }    .drop-down-content li:hover a{        background-color: red;    }    .nav .drop-down:hover .drop-down-content{        opacity: 1;        height: 150px;    }</style><ul class="nav">    <li class="drop-down">        <a href="#">下拉菜單</a>        <ul class="drop-down-content">            <li><a href="#">菜單1</a></li>            <li><a href="#">菜單2</a></li>            <li><a href="#">菜單3</a></li>        </ul>    </li></ul><p>內(nèi)容</p>