FWIW, I have an experimental open-source implementation of offline-capable PWA protomaps. Haven't advertised it much, because I haven't sorted out performance (particularly on mobile), and it's a bit buggy still. Also, don't have cache invalidation. But it does work. Usually. xD

app: https://maps.bpev.me

source: https://tangled.org/bpev.me/maps

Based on results from indexeddb pmtiles work:

https://github.com/jtbaker/pmtiles-offline

https://github.com/protomaps/PMTiles/issues/395