关于div水平垂直居中的几种方法

Dom结构：

<div class="box">
        <div class="inner">
            123
        </div>
</div>

1， 伪元素 加 vertical-align 

     　　.box {
            width: 300px;
            height: 300px;
            border: 1px solid red;
            text-align: center;
        }
        .box:before{
            content: '';
            height: 100%;
            width: 0;
            display: inline-block;
            vertical-align: middle;
        }
        .inner{
            display: inline-block;
        }

2， table -ceil

        .box {
            width: 300px;
            height: 300px;
            border: 1px solid red;
            display: table-cell;
            vertical-align: middle;
            text-align: center;
        }    

3， flex

         .box {
            width: 300px;
            height: 300px;
            border: 1px solid red;
            display: flex;
            justify-content: center;
            align-items: center;
        }
    

4， transform  

        .box {
            width: 300px;
            height: 300px;
            border: 1px solid red;
            position:relative;
        }
        .inner{
            position: absolute;
            top: 50%;
            left: 50%;
            transform: translate(-50%, -50%);
        }

5， absolute + margin：auto  （子元素要设置宽高）

        .box {
            width: 300px;
            height: 300px;
            border: 1px solid red;
            position:relative;
        }
        .inner{
            position: absolute;
            top: 0;
            left: 0;
            right: 0;
            bottom: 0;
            width: 100px;
            height: 100px;
            margin: auto;
        }

6， flex + margin：auto

        .box {
            width: 300px;
            height: 300px;
            border: 1px solid red;
            display: flex;
        }
        .inner{
            margin: auto;
        }