Can an online tool work without internet?

Continuing after the connection drops proves only that the required code and data are already loaded; it does not prove that nothing was transmitted earlier. For a stronger test, open the browser's network panel, reload the tool, observe requests, then disconnect and reload again from a closed tab. A service worker or installed web app may provide a complete offline shell, while an ordinary cached page may fail on the next visit. Save critical inputs and outputs locally instead of assuming the browser cache is durable storage.