实现思路如下：
1. div初始居普通文档流中
2. 给window添加scroll事件（可事件节流），获取div的offset的top值，滚动时scrollTop值和top比较，当到达top时给div添加一个fixed的class使其固定
3. 向上滚动时当到达div初始top时则删除fixed的class，此时div又回到普通文档流中
4. fixed样式非IE6浏览器使用position:fixed，IE6使用position:absolute和IE expression
效果图：
代码如下：
<Meta charset="utf-8">
无<a href="/tag/biaoti/" target="_blank" class="keywords">标题</a>文档
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