• Gnome Devhelp²
• Zeal³
• Local RFC archive⁴ dumps (by Debian⁵)
• Man pages
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