今天来个3D旋转菜单，是纯css3实现的，主要用到transform,transition,backface-visibility。
主要是transform这个变换，它是今天猪脚。
transform里有transform-style 属性规定如何在 3D 空间中呈现被嵌套的元素。
transform-origin 属性允许您改变被转换元素的位置。
backface-visibility 属性定义当元素不面向屏幕时是否可见。
代码：
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title>Document</title>
<style>
.nav{margin:0;
padding:50px;}
.nav li{float:left;
list-style:none;
padding:0 15px;
-webkit-perspective:100px;}
.nav a{float:left;
position:relative;
font:14px/20px "宋体";
text-decoration:none;
-webkit-transform-style:preserve-3d;
transition:.5s;
-webkit-transform-origin:center center -10px;}
.nav a:hover{
-webkit-transform:rotateX(-90deg);}
.nav span{float:left;
width:60px;
text-align:center;
-webkit-backface-visibility:hidden;}
.nav span:nth-of-type(2){
position:absolute;
left:0;
top:-20px;
-webkit-transform-origin:bottom;
-webkit-transform:rotateX(90deg);} </style>
</head>
<body>
<ul class="nav">
<a href="#" rel="external nofollow" rel="external nofollow" rel="external nofollow" rel="external nofollow" >
<span>选项一</span>
<span>选项1</span>
</a>
<a href="#" rel="external nofollow" rel="external nofollow" rel="external nofollow" rel="external nofollow" >
<span>选项二</span>
<span>选项2</span>
</a>
<a href="#" rel="external nofollow" rel="external nofollow" rel="external nofollow" rel="external nofollow" >
<span>选项三</span>
<span>选项3</span>
</a>
<a href="#" rel="external nofollow" rel="external nofollow" rel="external nofollow" rel="external nofollow" >
<span>选项四</span>
<span>选项4</span>
</a>
</ul>
</body>
</html>
