图片演示：
鼠标放到一级菜单栏时，二级菜单则会淡出并且从上至下依次反转。鼠标一离开则二级菜单会淡入，并且也会反转。
代码演示：
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title>国民老实人九轩博客www.jiuxuan.xyz-二级菜单淡入淡出反转动画实现</title>
<style>
*{
margin: 0;
padding: 0;
}
.nav{
width: 400px;
height: 40px;
margin: 0 auto;
margin-top: 100px;
background-color: black;
}
.nav>li{
list-style: none;
float: left;
width: 120px;
height: 40px;
background-color: green;
margin-left: 10px;
line-height: 40px;
text-align: center;
position: relative;
/*position: relative;相对定位*/
}
/*.sub{*/
/*!*display: none;不显示元素，菜单栏中比较关键的*!*/
/*display: none;*/
/*}*/
.sub{
width: 120px;
position: absolute;
/*position: absolute;绝对定位*/
left: 0;
top: 40px;
}
.sub li{
list-style: none;
background-color: pink;
transform: rotateY(180deg);
transition: all 1s;
opacity: 0;
/*opacity: 1px;透明度*/
}
/*.nav>li:hover .sub{*/
/*!*可以实现二级菜单的展出，但是动画效果就消失了*!*/
/*display: block;*/
/*}*/
.nav>li:hover .sub li{
transform: none;
opacity: 1;
}
.nav>li:hover .sub li:nth-child(1){
transition-delay: 0ms;
}
.nav>li:hover .sub li:nth-child(2){
transition-delay: 200ms;
}
.nav>li:hover .sub li:nth-child(3){
transition-delay: 400ms;
}
.nav>li:hover .sub li:nth-child(4){
transition-delay: 600ms;
}
.nav>li:hover .sub li:nth-child(5){
transition-delay: 800ms;
}
.nav>li .sub li:nth-child(5){
transition-delay: 0ms;
}
.nav>li .sub li:nth-child(4){
transition-delay: 200ms;
}
.nav>li .sub li:nth-child(3){
transition-delay: 400ms;
}
.nav>li .sub li:nth-child(2){
transition-delay: 600ms;
}
.nav>li .sub li:nth-child(1){
transition-delay: 800ms;
}
div{
width: 400px;
height: 300px;
background-color: red;
margin: 0 auto;
}
</style>
</head>
<body>
<ul class="nav">
<li>一级菜单
<ul class="sub">
<li>二级菜单</li>
<li>二级菜单</li>
<li>二级菜单</li>
<li>二级菜单</li>
<li>二级菜单</li>
</ul>
</li>
<li>一级菜单
<ul class="sub">
<li>二级菜单</li>
<li>二级菜单</li>
<li>二级菜单</li>
<li>二级菜单</li>
<li>二级菜单</li>
</ul>
</li>
<li>一级菜单
<ul class="sub">
<li>二级菜单</li>
<li>二级菜单</li>
<li>二级菜单</li>
<li>二级菜单</li>
<li>二级菜单</li>
</ul>
</li>
</ul>
<div>
我是文字
</div>
</body>
</html>
