is there a standard of proof these sites can provide about not sending data back to the servers? besides checking the network data, which it could delay to another visit or obfuscate easily
I mean, you can be completely sure that the website works offline by unplugging the cable/turning off your wifi connection after it's completely loaded. But that's just a functional test, not something you can expect users to do during a normal browsing session
I'd like some API for a page to load, then become "offline". Then I can use it, and have the browser block any attempts to send/receive data from anywhere except local storage.
If you're worried about that, loading it in a private window and switching to offline mode, then closing it after you're done makes any exfiltration impossible.