You can download the app on your mobile device or desktop app for macOS in order for the site to work without the Internet. You can also save the web resources and run them locally.
Just use the OpenSprinkler mobile app — it has all the necessary Javascript built-in so can show the UI/homepage without internet.
Also I’ve often received questions about how to use OS 3 in AP mode without Internet, examples would be a remote area with no nearby service providers. As long as you use the mobile app, the controller can stay in AP mode and your phone can talk to it through the mobile app.