Caliu works fully offline. Your notes are stored on your device, so no connection means no difference: you can read, write, search, tag, pin, archive, delete, and export exactly as you would online.
What happens to your changes
Every change you make offline is saved to your device instantly and added to a local queue. The moment you are back online, the queue syncs to the server automatically. There is nothing to do and nothing to remember.
Even images work: paste one into a note while offline and it shows up immediately. The upload happens later, in the background, once you reconnect.
The sync indicator in the sidebar shows "Offline" along with how many changes are waiting. When the connection returns, they flush on their own.
The web app offline
The web app is a Progressive Web App. Once you have signed in on a device, it starts and loads your notes even with zero connectivity. You can also install it from your browser so it behaves like a native app, icon and all.
One caution
Logging out discards changes that have not synced yet. If you have pending changes, Caliu warns you before you log out. Connect and let the sync finish first.