<script type="text/javascript">// <![CDATA[ 
$.fn.smartFloat = function() { 
  var position = function(element) { 
    var top = element.position().top, pos = element.css("position"); 
    $(window).scroll(function() { 
      var scrolls = $(this).scrollTop(); 
      if (scrolls > top) { 
        if (window.XMLHttpRequest) { 
          element.css({ 
            position: "fixed", 
            top: "10px" 
          }); 
        } else { 
          element.css({ 
            top: scrolls 
          }); 
        } 
      }else { 
        element.css({ 
          position: pos, 
          top: top 
        }); 
      } 
    }); 
  }; 
  return $(this).each(function() { 
    position($(this)); 
  }); 
}; 
 
//绑定 
$("#float").smartFloat(); 
// ]]></script> 

/*页面智能层浮动*/ 
jQuery(document).ready(function($){ 
  var $sidebar = $(".float"), 
  $window = $(window), 
  offset = $sidebar.offset(), 
  topPadding = 20; 
  $window.scroll(function() { 
    if ($window.scrollTop() > offset.top) { 
      $sidebar.stop().animate({ 
        marginTop: $window.scrollTop() - offset.top + topPadding 
      }); 
    } else { 
      $sidebar.stop().animate({ 
        marginTop: 0 
      }); 
    } 
  }); 
}); 

<div id="float" class="float"> 
<h3>推荐</h3> 
广告代码 
</div>