Is it possible to implement workbox (https://developers.google.com/web/tools/workbox/) in an application without node / npm (html + js + css)? I still have not found this answer ...
It looks like, yes, it should work just fine without using Node itself. There is also a CDN hosted version, so you won't need to use NPM to download it either.
If you check out their Get Started page, it'll show a simple starting example which just uses basic browser stuff.