.banner{position: relative;overflow: auto;text-align: center; width:100%; margin:0 auto}
.banner .arrow{position:absolute;width:20px;height:37px;top: 50%; margin-top: -18px; overflow: hidden;z-index: 2; -webkit-transition: all 0.5s; -moz-transition: all 0.5s;  -ms-transition: all 0.5s; -o-transition: all 0.5s; transition: all 0.5s;}
.banner #al{left:15px}
.banner #ar{right:15px}