元素水平垂直居中 ​

定位+margin：auto ​

<style>
  .father {
    width: 500px;
    height: 300px;
    border: 1px solid #0a3b98;
    position: relative;
  }
  .son {
    width: 100px;
    height: 40px;
    background: #f0a238;
    position: absolute;
    top: 0;
    left: 0;
    right: 0;
    bottom: 0;
    margin: auto;
  }
</style>
<div class="father">
  <div class="son"></div>
</div>

定位+transform ​

<style>
  .father {
    position: relative;
    width: 200px;
    height: 200px;
    background: skyblue;
  }
  .son {
    position: absolute;
    top: 50%;
    left: 50%;
    transform: translate(-50%, -50%);
    width: 100px;
    height: 100px;
    background: red;
  }
</style>
<div class="father">
  <div class="son"></div>
</div>

table 布局 ​

设置父元素为 display:table-cell，子元素设置 display: inline-block。利用 vertical 和 text-align 可以让所有的行内块级元素水平垂直居中

<style>
  .father {
    display: table-cell;
    width: 200px;
    height: 200px;
    background: skyblue;
    vertical-align: middle;
    text-align: center;
  }
  .son {
    display: inline-block;
    width: 100px;
    height: 100px;
    background: red;
  }
</style>
<div class="father">
  <div class="son"></div>
</div>