div在父元素内上下左右居中

     .box {
            width: 400px;
            height: 200px;
            position: relative;
            background: red;
        }
        .content {
            width: 200px;
            height: 100px;
            position: absolute;
            top: 50%;
            left: 50%;
            margin-left: -100px;
            margin-top: -50px;
            background: green;
        }
         .box {