.mycss{
300px;
height:200px;
position:absolute;
left:50%;
top:50%;
margin:-100px 0 0 -150px }
CSS代码：
.mycss{
position: absolute;
left: 0px;
right: 0;
top: 0;
bottom: 0;
margin: auto;
height: 200px;
300px;
}