div在页面垂直居中方法---增强改进版

.wrap{
    background: #ffffff;
    position:absolute;
    margin:auto;
    top: 0;
    bottom: 0;
    left:0;
    right: 0;
    height: 630px;
}

.wrap{
    background: #ffffff;
    position:absolute;
    margin:auto;
    top: 0;
    /*bottom: 0;*/
    left:0;
    right: 0;
    height: 630px;
}
@media screen and (min-height: 630px) {
    .wrap {
        bottom: 0;
    }
}