css clear 清除浮动
.clearfix:after {content:'';display:block;height:0;overflow:hidden;clear:both;} IE8以上的浏览器
.clearfix {*zoom:1} IE6/IE7
.clearfix:after {content:'';display:block;height:0;overflow:hidden;clear:both;} IE8以上的浏览器
.clearfix {*zoom:1} IE6/IE7