If you replace the "x" with a "5" in an ArXiv URL, you get an HTML version

Are there any similarly benign / beneficial examples you've come across for other sites?

Not really a typo, but you can replace fandom.com with antifandom.com on any Fandom site to get an unshittified version.

It produces pretty decent results; not always the best, but it makes browsing papers on the web more seamless.

Arxiv now supports HTML natively. Not sure if the quality is different or it is now supported for all old articles.

It is experimental and doesn't work on all PDFs.

Nothing will work on all PDFs. A PDF could order its contents randomly letter by letter and then just position them right.

They're both using LaTeXML v0.8.8, so both will output the same HTML.

Example: