纯CSS实现下拉菜单

将下拉菜单的ul高度设置为0，并且超出部分隐藏掉。

设置下拉菜单的高度添加过渡效果，高度为auto时过渡效果失效。

<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="#">下拉菜单</a>
        <ul class="drop-down-content">
            <li><a href="#">菜单1</a></li>
            <li><a href="#">菜单2</a></li>
            <li><a href="#">菜单3</a></li>
        </ul>
    </li>
</ul>
<p>内容</p>

效果图如下：