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I already downloaded documentations, like the python api, or the cpp preference website as a pdf or html archive.

I don't know if it's available for html or js or css, or opengl.

You should check out Zeal, it's an offline documentation browser with existing documentation packages for HTML and a whole bunch of things

https://zealdocs.org/

https://devdocs.io/ exposes a huge catalog of indexed and searchable collections of documentation for a wide variety of languages, libraries, and subjects, including HTML, JS, and CSS – though, the only GL I see is WebGL – and _all_ of it can be downloaded to an IndexedDB for offline use.

It's been a very handy tool in my toolbelt.

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