I like GNU documentation (texinfo): reading those both as info manuals (usually in Emacs) and as HTML (in a web browser). Nice search and convenient navigation in an info viewer, available (and convenient) both offline and online, hyperlinks, HTML versions don't rely on JS (and do try to stay compatible with ancient web browsers), and it's rather generic (not language-specific).

Edit: I think pretty much all the GNU documentation (and most of texinfo documentation) I saw was nice, but as particular and common examples, there are manuals for Emacs and Elisp, GNU libc, coreutils, bash.