For offline documentation, I use these in order of preference:

• Info¹ documentation, which I read directly in Emacs. (If you have ever used the terminal-based standalone “info” program, please try to forget all about it. Use Emacs to read Info documentation, and preferably use a graphical Emacs instead of a terminal-based one; Info documentation occasionally has images.)

• Gnome Devhelp².

• Zeal³

• RFC archive⁴ dumps provided by the Debian “doc-rfc“ package⁵.

1. https://www.gnu.org/software/emacs/manual/html_node/info/

2. https://wiki.gnome.org/Apps/Devhelp

3. https://zealdocs.org/

4. https://www.rfc-editor.org/

5. https://tracker.debian.org/pkg/doc-rfc