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Sometimes you look at an old HTML document and see some weird tags and snippets from bygone eras. I've complied many legacy HTML bits I've noticed and documented what they were and why there were present. Many of them are rather obscure. Satisfy your curiosity: vale.rocks/posts/html-r...

meta keywords is another one for your list.

I did consider including meta keywords, but they're still very loosely relevant in some cases. I believe Baidu, Sogou, Sogou, and Yandex still use them (albeit very lightly) for weighting. I also believe some niche independent search engines use them too, as well as some web ring projects.