看了一些技术类网站，为啥很多都加了 nofollow?

例如：

v2ex: <a href="https://jiajunhuang.com" class="social_label" target="_blank" rel="nofollow"><img src="/static/img/social_home.png" alt="Website" width="24" align="absmiddle">  https://jiajunhuang.com</a>
csdn: <a href="https://jiajunhuang.com/articles/2019_06_11-frpc_source_code_part1.md.html" rel="nofollow" target="_blank">https://jiajunhuang.com/articles/2019_06_11-frpc_source_code_part1.md.html</a>
juejin: <a target="_blank" href="https://link.juejin.im?target=https%3A%2F%2Fgithub.com%2Fhoc2019%2Fblog" rel="nofollow noopener noreferrer">github.com/hoc2019/blo …</a>
segmentfault: <a href="https://github.com/yulonghu/sg" rel="nofollow noreferrer" target="_blank">https://github.com/yulonghu/sg</a>
zhihu: <a href="https://link.zhihu.com/?target=http%3A//www.cnfeat.com/blog/2014/05/11/how-to-build-a-blog/" class=" wrap external" target="_blank" rel="nofollow noreferrer" data-za-detail-view-id="1043">如何搭建一个独立博客--简明 GitHub Pages 与 jekyll 教程</a>

也有没加的，例如：

当然，国外的暂时只看了 GitHub、Hacker News、stackoverflow 和维基百科：

github: <a rel="nofollow me" href="https://jiajunhuang.com">https://jiajunhuang.com</a>
stackoverflow: <a href="http://golang.org/pkg/syscall/" rel="nofollow noreferrer"><code>syscall</code></a>

HN，维基百科没加。

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