It should work. I've tried it many times in the past and verified it does work offline.
If it got somehow broken at some point, please file an issue following all the guidelines. Please note any report about C3 not working offline must start with these steps:
1. Clear storage/browser data/etc to revert to clean (uncached) state
2. Load C3 while online and wait for the "ready to use offline" notification.
3. (Whatever further steps necessary to reproduce the problem)
Several times in the past people have skipped step 2 and as far as I saw it was working fine.