用纯css代码写一个css下拉菜单，二级菜单opacity默认是0，鼠标在一级菜单hover时opacity就是1，opacity从0到1有一个transition过渡时间1秒钟，就实现了css下拉菜单动画缓慢出现效果，具体看下面例子代码。

css下拉菜单代码：

<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: #E0A800;

}



ul li a.menu {

background: #000000;

}

.nav {

width: 500px;

margin: 100px auto;

display: flex;

}

.drop-down {

width: 100px;

height: 50px;

margin-right: 30px;

}



.drop-down-content {

opacity: 0;

height: 0;

overflow: hidden;

transition: all 1s ease;

}



.drop-down-content li:hover a {

background-color: red;

}



.nav .drop-down:hover .drop-down-content {

opacity: 1;

height: 160px;

}

</style>

<ul class="nav">

<li class="drop-down">

<a class="menu" href="#">水果</a>

<ul class="drop-down-content">

<li><a href="#">苹果</a></li>

<li><a href="#">香蕉</a></li>

<li><a href="#">冬枣</a></li>

<li><a href="#">雪梨</a></li>

</ul>

</li>

<li class="drop-down">

<a class="menu" href="#">蔬菜</a>

<ul class="drop-down-content">

<li><a href="#">芹菜</a></li>

<li><a href="#">山芋</a></li>

<li><a href="#">青菜</a></li>

<li><a href="#">南瓜</a></li>

</ul>

</li>

</ul>

 原文地址：css下拉菜单代码(动画缓慢出现效果)