[wat10000]

Better yet, just save. Storage is cheap and fast these days. The “do you want to save?” idiom is a leftover from the days when a moderately sized document would take a noticeable amount of time to save and eat up a decent chunk of your floppy disk.

[tripflag]

But what if you are leaving the page because you changed your mind, and don't wish to save the changes after all? This, for me, is the common case, so i would not want the browser to suddenly commit an unfinished draft.

[wat10000]

If you’re worried about losing the old version, it should keep a history. If you want to erase the new version, there should be an explicit action to do that.

[Sayrus]

An unfinished upload or sync stored locally instead of on the remote can absolutely be an issue. You can look at all the posts about OneDrive and GDrive not actually syncing before confirming to users who then delete their files since they "have been uploaded". Or the user may never open that specific page again or the session will not exist anymore when he comes back.

Browser storage is cheap, but it is not guaranteed to be durable.

[wat10000]

That's a good point. You'd want some (overridable) way to block navigating away until the sync completed, or do it in the background. Local storage is pretty easy, remote can get tricky.