To allow safety and data security in the web for users, I propose that we add Offline/SandBoxed mode to JavaScript and/or browser. When SandBoxed mode is on for a website when enabled, it will only allow specific configured hosts or none (e.g. CSV file) for requests. The user could force a web application to be SandBoxed by enabling a proposed 'sandboxed' checkbox + config textbox/file when installing PWA of the website as well for example.
We can also allow inter-browser communication for SandBoxed applications to allow Syncing across devices as well.
This would allow user to ensure that data and interaction with web application is secure and available locally and not sent to any server.
This would allow, for example, creating notes or word application that saves documents only on users computer and not send any user data to cloud unless specified.