1.已知盒子的寬高
.box{
position: absolute;
left:50%;
top:50%;
width: 100px;
height: 100px;
margin-left: -50px;
marigin-top: -50px;
}
2.未知盒子的寬高
.box{
position: absolute;
left:50%;
top:50%;
transform : translate(-50%,-50%);
