backface-visibility:hidden；css3新属性
backface-visibility 属性定义当元素背面向屏幕时是否可见。
如果在旋转元素不希望看到其背面时，该属性很有用。
<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8" />
<meta name="viewport" content="width=device-width, initial-scale=1" />
<title></title>
<style>
body{
margin: 0;
}
.box{
margin: 200px auto;
position: relative;
width: 250px;
perspective:800px;
transform-style: preserve-3d;
}
.front{
position: relative;
text-align: center;
line-height:200px ;
width: 250px;
height: 200px;
background-color: red;
border-radius: 5px;
z-index: 1;
transition: all 2s;
backface-visibility:hidden;
}
.back{
position: absolute;
top: 0;
left: 0;
text-align: center;
line-height:200px ;
width: 250px;
height: 200px;
background-color: blue;
border-radius: 5px;
transform: rotateY(180deg);
backface-visibility:hidden;
transition: all 2s;
z-index:0;
}
.box:hover .front{
transform: rotateY(-180deg);
/* z-index: 1; */
}
.box:hover .back{
transform: rotateY(0deg);
}
</style>
</head>
<body>
<div class="box">
<div class="front">前面</div>
<div class="back">后面</div>
</div>
</body>
</html>