css使子元素在父元素居中的各种方法

html结构:

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

方法一： display:flex

  .parent {
     500px;
    height: 500px;
    background: red;
    display: flex;
    align-items: center;
    justify-content: center;
  }
  .child {
     100px;
    height: 100px;
    background: blue;
  }

方法二：display:table-cel

  .parent{
     500px;
    height: 500px;
    background: red;
    display: table-cell;
    vertical-align: middle;
  }
  .child{
     100px;
    height: 100px;
    background: blue;
    margin: auto;
  }

方法三：绝对定位和0

  .parent{
     500px;
    height: 500px;
    background: red;
    position: relative;
  }
  .child{
     100px;
    height: 100px;
    background: blue;
    margin: auto;
    position: absolute;
    top: 0;
    left: 0;
    bottom: 0;
    right: 0;    
  }

方法四：负边距

  .parent{
     500px;
    height: 500px;
    background: red;
    position: relative;
  }
  .child{
     100px;
    height: 100px;
    background: blue;
    position: absolute;
    top: 50%;
    left: 50%;
    margin-left: -50px;
    margin-top: -50px;  
  }

以上四种方法都可以完成用css实现子元素在父元素实现水平和垂直居中。