Offline docs are good.
Example:
rustup doc --std
Opens the local path to the docs in your web browser. No server. Everything works, even search.
A dev environment is more complete if functional offline.
I refrain from saying something like "any good dev environment can run entirely offline", because I've experience cloud-native projects where the nature of the work is simply that there are not guaranteed-compatible offline analogs to every component you are using and thus need to develop against. (Cloudflare has a brilliant offline mode for their CLI that simulates a chunk of their cloud systems sufficiently for some development.)
Rather, a more modest test, being able to develop offline is one eligible sign of a well-managed project.
Similarly, being able to develop offline is one eligible sign of a skilled engineer.
Of course, one can easily be skilled and effective as an engineer while nonetheless being approximately dead-in-the-water without ... uhh... grr... I was going to say Stack Overflow, RIP, but now it is your preferred LLM (which may be run locally!)... or at least Google Search for rapid access to the relevant docs pages of the two dozen libraries and frameworks you may be simultaneously layering into your project.
Perhaps an eligible test for whether to include a third-party library is: Would you bother yourself to download a copy of this tool's documentation?