However, I am not a fan of uploading databases to "strange" sites on the internet, so I will probably never use this.
I loaded up my Star Trek database into the program to toy with: https://chiselapp.com/user/chungy/repository/startrek-db/uvl...
1. Permanently restrict the browser tab from accessing the network.
2. Show an indication of this in the browser UI outside of the content area.
It would be perfect for this kind of app.
But you can get sort of what you want by:
1. Loading a site.
2. Go into the Web Dev Tools.
3. Select the Network tab.
4. Click on the "No Throttling" dropdown and check "Offline".
5. You should now see a yellow warning icon in your Network tab and it should prevent all network access. If you reload the tab you should get the "No Internet Connection" error page.
The pledge would have to be persistent, which would make e.g. service worker updates difficult - ideally the browser should show you that an update is available, and offer to delete all stored information so nothing can be saved to localStorage etc. to be uploaded afterwards.