clearfix 兼容IE6/IE7，解决ie6/ie7下多出一行的问题，bootstrap的clearfix的bug

         .clearfix:before,
        .clearfix:after {
          content: ".";
          display: block;
          height: ;
          overflow: hidden;
        }

        .clearfix:after {
            clear: both;
        }

        .clearfix {
            zoom: ; /* IE < 8 */
        } 

来源：http://www.css-101.org/articles/clearfix/latest-new-clearfix-so-far.php