margin: auto;
position: absolute;
top: 50%;
left: 50%;
-webkit-transform: translate(-50%,-50%);
-ms-transform: translate(-50%,-50%);
transform: translate(-50%,-50%);
50%定位,在平移50%
margin: auto;
position: absolute;
top: 50%;
left: 50%;
-webkit-transform: translate(-50%,-50%);
-ms-transform: translate(-50%,-50%);
transform: translate(-50%,-50%);
50%定位,在平移50%