容器居中

1、margin: auto

div {
  position: absolute;
  top: 0;
  left: 0;
  right: 0;
  bottom: 0;
  margin: auto;
}

2、transform

div {
  position: absolute;
  top: 50%;
  left: 50%;
  transform: translate(-50%, -50%);
}

3、容器高、宽已知，margin

div {
  width: 400px;
  height: 400px;
  position: absolute;
  top: 50%;
  left: 50%;
  margin: -200px -200px;
}