The UI mostly works offline once loaded in due to aggressive caching.
Downloading Course Content was on the initial Roadmap but I removed it since I wasn't sure if anyone would like the feature.
Syncing stuff is a real pain in the ass but I'll implement it if at least a couple people want it.
I don't know how much of a subset I am, but I still use dictionary softwares from Windows 95~2000 era and Android softwares on a completely offline and vanilla VM, partly due to internet randomly going bad, and partly because I am neurotic about digital contents vanishing once support ends.