Would be nice to have the option to switch tabs into offline mode, just like we can mute them.

You can do that with Chrome dev tools: Network -> No throttling -> Offline

Don't know how reliable this is though or whether a web developer could work around this.

It could be storing data and send it later when you visit the same origin from a different tab.

Unless you were also using incognito or throwaway tab containers to discard stored data.

Good point, thanks.

Turn off wifi? Unplug the ethernet cable? Try it from my garden shed where there never seems to be connectivity no matter what I try.

I think there is an easy way to prove this to users. Make your thing be a single page self contained html file which they save into the hard disk. Then they can trust the restricted permissions with which chrome runs such local files.

If you have a tech savvy audience they can also view your thing in an iframe with only sandbox="allow-scripts" to prove that it's not making network requests.

I wrote an html/js log viewer with those security models https://GitHub.com/ljw1004/seaoflogs - it handles up to 10kline log files decently, all locally.