CSS：div 水平垂直居中

flex

<div class="parent">
  <div class="child"></div>
</div>

.parent {
  display: flex;
  align-items: center;
  justify-content: center;
  height: 100vh;
  width: 100wh;
}

.child {
  width: 100px;
  height: 100px;
  background-color: #20126f;
}