I see. The case in question is when the user switches between two devices, each of which may be sometimes offline. This can cause older changes hitting the server later than the new ones, in which case, ideally, they should be rejected.

What happens to storage requests made while offline? Are they cached locally and propagated once online, or are they lost?

I feel like, if the answers to these questions are important, you should be using a more complex solution.

Users can have an app in multiple tabs though, and apps can read/write local storage in the background.

The funny thing is that localStorage supports that.

Judging by the title, I thought this tool actually just synced the existing localStorage instead of being its own storage. Since localStorage has onChange events it would be trivial to built, it would still be sync, it wouldn’t require any changes to the existing code, etc