CSS实现子元素div水平垂直居中的示例

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

.container {
  display: flex;
  justify-content: center;
  align-items: center;
  height: 300px;
  border: 1px solid black;
}

.child {
  width: 100px;
  height: 100px;
  background-color: red;
}

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

.container {
  position: relative;
  width: 300px;
  height: 300px;
  background-color: gray;
}

.child {
  position: absolute;
  left: 50%;
  top: 50%;
  transform: translate(-50%, -50%);
  width: 100px;
  height: 100px;
  background-color: red;
}