position 上下左右垂直居中

.login{
  width: 100%;
  height: 100%;
  position: absolute;
  top: 0;
  left: 0;
  background: rgba(0,0,0,.3);
  .log{
    width: 425px;
    height: 600px;
    background: #FFF;
    position: absolute;
    top: 50%;
    transform: translateY(-50%);
    left: 0;
    right: 0;
    margin: 0 auto;
  }
}