3種水平垂直的方式

1 絕對定位 + 轉換

<div class="parent">
  <div class="child">絕對定位 + 轉換</div>
</div>

.parent {
  position: relative;
  width: 400px;
  height: 400px;
  background: skyblue;
}
.child {
  position: absolute;
  left: 50%;
  top: 50%;
  transform: translate(-50%, -50%);
  width: 200px;
  height: 200px;
  background: pink;
}

2 彈性模型

<div class="parent">
  <div class="child">彈性模型</div>
</div>

.parent {
  display: flex;
  justify-content: center;
  align-items: center;
  width: 400px;
  height: 400px;
  background: skyblue;
}
.child {
  width: 200px;
  height: 200px;
  background: pink;
}  

3 單元格方式

<div class="parent">
  <div class="child">單元格方式</div>
</div>  

.parent {
  display: table-cell;
  text-align: center;
  vertical-align: middle;
  width: 400px;
  height: 400px;
  background: skyblue;
}
.child {
  display: inline-block;
  width: 200px;
  height: 200px;
  background: pink;
}