点击左上角的按钮前后切换
效果图：
代码如下：
<!doctype html>
<html>
<head>
<meta charset=UTF-8>
<title>Title</title>
<style>
*{margin:0; padding:0; list-style:none;}
ul{
width:300px;
height:300px;
margin:100px auto;
position:relative;
-webkit-transform-style:preserve-3d;
perspective:800px;
}
ul li{
position
