在下面的演示代码中，用到的html结构为：

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

方法一：positoin+margin

        .content{
            height: 200px;
            width: 200px;
            background-color: #ccc;
            position: relative;
        }
        .center{
            width: 100px;
            height: 100px;
            background-color: pink;
            margin: auto;
            position:absolute;
            left: 0;
            top: 0;
            right: 0;
            bottom: 0; 
        }

方法二：display：tell-cell;

.content{
            height: 200px;
            width: 200px;
            background-color: #ccc;
            display: table-cell;
            vertical-align: middle;
            text-align: center;
        }
        .center{
            display: inline-block;
            width: 100px;
            height: 100px;
            background-color: pink;
            vertical-align: middle;
            
        }

方法三：display：flex;align-items:center；justify-content：center；

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

方法四：display：flex；margin:auto;(主要适用移动端)

          .content{
            height: 200px;
            width: 200px;
            background-color: #ccc;
            display: flex;
        }
        .center{
            background-color: pink;
               width: 100px;
            height: 100px;
            margin: auto;
        }

方法五：纯position

          .content{
            height: 200px;
            width: 200px;
            background-color: #ccc;
            position: relative;
        }
        .center{
            background-color: pink;
             width: 100px;
            height: 100px;
            position: absolute;
            left: 50px;
            top: 50px;
        }

content{
height: 200px;
width: 200px;
background-color: #ccc;
position: relative;
}
.center{
background-color: pink;
width: 100px;
height: 100px;
position: absolute;
left: 50%;
margin-left: -50px;//-1/2宽度
top: 50%;
margin-top: -50px;//-1/2高度
}