记几种居中的方法

1. 定位方法

.child{
            width: 200px;
            height: 200px;
            position: absolute;
            background: #000;
            left: 0;
            top: 0;
            right: 0;
            bottom: 0;
            margin: auto;
        }

2.基于定位和位移

.content{
            width: 200px;
            height: 200px;
            position: absolute;
            left: 50%;
            top: 50%;
            -webkit-transform: translate(-50%,-50%);
            -moz-transform: translate(-50%,-50%);
            -ms-transform: translate(-50%,-50%);
            -o-transform: translate(-50%,-50%);
            transform: translate(-50%,-50%);
            background: #000;
        }

3.基于定位和margin负值

        .content{
            width: 200px;
            height: 200px;
            position: absolute;
            left: 50%;
            top: 50%;
            margin-left:-100px;
            margin-top:-100px;
            background: #000;
        }

4.flex布局方法，也是几种方法中最优的办法，定位可能会影响到结构布局。

.wrapper{
            width: 500px;
            height: 500px;  
            display: -webkit-box; //felx兼容写法
            display: -webkit-flex;
            display: -moz-flex;
            display: -ms-flexbox;
            display: flex;
            align-items: center;
            justify-content: center;
            flex-direction: column;
        }

也可以这样写

.wrapper{
            width: 500px;
            height: 500px;
            display: -webkit-box;
            display: -webkit-flex;
            display: -moz-flex;
            display: -ms-flexbox;
            display: flex;
        }
        .content{
            width: 200px;
            height: 200px;
            margin: auto;
            background: #000;
        }

参考文献：https://caniuse.com/#search=flex
https://blog.youkuaiyun.com/freshlover/article/details/11579669