I made a proof of concept app that is quite handy and works online or offline: https://foodflight.app/
I can go to the site and turn off wifi, go the page, look up contents of foods.
Unfortunately on mobile Firefox-fennec, I cannot. It shows no connection when trying to go to the url.
Chrome mobile has an even weirder problem, although I can view the page offline, all form inputs seem to be disabled for some reason. Connecting and testing F12 tools of desktop Chrome crashes it.