Emacs has self-documentation as one of its goals, and it achieves this pretty well for Emacs Lisp with commands like
describe-function, etc. Packages like company-quickhelp make this even more convenient by providing help in tooltips, which reduces the need to constantly switch attention to a Help buffer.
The built-in Info reader is also very convenient for browsing Info manuals.
Helm improves upon this by providing helm-info, which allows for choosing from all installed Info files and searching a file's index via incremental completion.
I find the above tools important because, by making documentation look-up easier, they encourage me to do more of it.
However, it's difficult to replicate the ease with which I can look up documentation about Emacs, Emacs Lisp, etc. with other tools and languages. For example, if I forget what a CSS rule does, I still find myself Googling for answers, which is slow and shifts attention away from Emacs.
So, what are some good ways (both general and specific to particular tools/languages) to look-up documentation or check manuals in Emacs?