1.利用flexbox布局

        .parent{
            width: 100%;
            height: 37.5rem/* 600px */;
            background: #09c;
            display: flex;
            justify-content:center;     /* 水平居中 */
            align-items:center;         /* 垂直居中 */
            /* flex-direction:column; */    /* 一列显示 */
        }
        .children{
            width: 100px;
            height: 100px;
            background-color: #eee;
            border: 1px dashed #000;
            margin: 5px;
            /*如果children下面还有子元素的话，可以嵌套使用*/
            /* display: flex;
            justify-content: center;
            align-items:center;  */
        }

2.利用绝对定位与transform

        .parent{
            position: absolute;
            background-color: #eee;
            width: 100%;
            height: 100%;
        }
        .parent .children{
            background-color: #751;
            width: 200px;
            height: 200px;
            position: absolute;
            top: 50%;
            left: 50%;
            -webkit-transform: translate(-50%,-50%);
        }

3.将父元素定位，子元素绝对定位，利用margin负值为子元素宽高的一半来实现。

        .parent{
            position: relative;
            background-color: #eee;
            height: 600px;
            width: 100%;
        }
        .parent .children{
            background-color: #751;
            width: 200px;
            height: 200px;
            position: absolute;
            top: 50%;
            left: 50%;
            margin: -100px 0 0 -100px;
        }

4.利用定位与margin：auto

        .parent{
            width: 100%;
            height: 37.5rem/* 600px */;
            background: #09c;
            position: relative;
        }
        .children{
            width: 100px;
            height: 100px;
            background-color: #eee;
            position: absolute;
            top: 0;
            left: 0;
            bottom: 0;
            right: 0;
            margin: auto;
        }