*{margin:0;padding:0;}
.box{width:200px;height:200px;border:1px solid #666;margin:40px;position:relative}
.b{display:none;}
.div{width:100%;height:100%;position:absolute;}
.div span{position:absolute;top:0;right:0;width:30px;height:30px;background:#fff;text-align:center;line-height:30px;font-size:18px;cursor: pointer}
.a{z-index:1;background:#eee;}
.b{z-index:2;background:#fff}