The only thing it could do is alarm you for navigating away from any webpage that has any form that has ever had input put into it. Also, the website could intercept the "go back" event for some purpose. Should the browser fire an alarm then, as well? Either way, it would be a terrible user experience.

There's no notion of "unsaved" changes. There's the notion of form data that hasn't been submitted, which is entirely different. Many web apps don't use "submit" at all, ever.