基础代码：

<div class="box">
  <div class="content"></div>
</div>

css实现居中的几种方式：

        1、flex布局（水平垂直）

.box {
  width: 200px;
  height: 200px;
  background-color: pink;
  display: flex;
  justify-content: center;
  align-items: center;
}
.content {
  width: 100px;
  height: 100px;
  background-color: lawngreen;
}

        2、margin:auto;（水平）

.box {
  width: 200px;
  height: 200px;
  background-color: pink;
}
.content {
  width: 100px;
  height: 100px;
  background-color: lawngreen;
  margin: auto;
}

        3、固定定位+transform（水平垂直）

.box {
  width: 200px;
  height: 200px;
  background-color: pink;
  position: relative;
}
.content {
  width: 100px;
  height: 100px;
  background-color: lawngreen;
  position: absolute;
  left: 50%;
  top: 50%;
  transform: translate(-50%, -50%);
}

        4、vertical-align: middle;（垂直）

.box {
  width: 200px;
  height: 200px;
  background-color: pink;
}
.content {
  width: 100px;
  height: 100px;
  background-color: lawngreen;
}
.box::after, .content{
  display:inline-block;
  vertical-align:middle;
}
.box::after{
  content:'';
  height:100%;
}

        5、line-heihgt + text-align

.text-box {
  width: 100px;
  height: 50px;
  background-color: aquamarine;
  text-align:center;
}
.text-box span {
  line-height: 50px;
}

<div class="text-box"><span>1</span></div>