rel="nofollow" 表示该链接不被推荐给搜索引擎，希望搜索引擎不要将该链接计入 Page Rank。
现在不少 Web 2.0 站点，给用户提供评论功能，比如论坛，博客评论等，有些人就故意发帖，放很多垃圾链接在上面，企图提高这些链接的 Page Rank。这就叫做 comment spam。
Google, Yahoo, MSN 支持 rel="nofollow"，一旦某个链接有rel="nofollow" 属性，搜索引擎就不会将其作为计算该链接 Page Rank 的考量值。
The nofollow keyword may be used with a and area elements.
The nofollow keyword indicates that the link is not endorsed by the original author or publisher of the page, or that the link to the referenced document was included primarily because of a commercial relationship between people affiliated with the two pages.
rel="external" 表示该链接不是本站的，而是其它站点的链接。
The external keyword may be used with a and area elements.
The external keyword indicates that the link is leading to a document that is not part of the site that the current document forms a part of.
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