Alt - F - K

(menubar File => Work offline)

It applies to the whole browser, not the current tab. But good enough.

Does this absolutely prevent a page from saving information, eg local stiarge, and later transmitting it when you either visit the site or a Web page where it is iframed?

I doubt it, but I bet you can combine this with a private window and everything gets wiped from loca storage after it's closed.

For that you may use containers, private mode, the menu to delete site data, and auto-delete.

So even here on HN, we've got a thread which:

1. starts by suggesting devTools, which is simple, elegant, and wrong.

2. improves by suggesting the "Work Offline" menu option of the File menu, which is hidden by default on both Windows and Linux, and also wrong.

3. improves to a state of minimal functionality with implicitly ordered steps to use Private Browsing, Offline Mode, and confidential file "upload." And, I guess, always remembering to close all private browsing windows upon completion?

I'll rankly speculate f this ever caught on, 25% of users will forget to check offline mode until after they have finished editing their confidential document, 40% will leave a stray Private Browsing window open at all times, 10% will accidentally continue doing all their browsing in the same Private Browsing window, and 1% will somehow paste their private GPG keys in the query string of the URL.

This far down the thread, we are all caught in the "how could we achieve this", but I think it's clear that it's not easy.

Websites are not meant to not use the internet.

Create a new profile from about:profiles to do this, and delete it afterwards.