浮动的盒子居中

    .parent {
        width: 450px;
        height: 500px;
        background: pink;
        position: relative;
    }
    .child {
        width: 200px;
        height: 200px;
        float:left;
        border: 1px solid #000;
        position: absolute;
        left:50%;
        top:50%;
        margin: -100px 0 0 -100px;
        background: gold;
    }