水平垂直居中

div{
    width:100px;
    height:100px;
    position:absolute;
    top:0;
    right:0;
    bottom:0;
    left:0;
    margin:auto;
}

父级控制子级居中

.parent{
    dsiplay:flex;
    justify-content:center;
    align-items:center;
}