[JoshTriplett]

Ad blockers have false positives as well. And there's a use case for blocking the user from closing the tab (onbeforeunload), such as prompting them to save/submit what they're working on. But for all of those, the browser is still in control and the question is what provides the most benefit for the user.

So, along the same lines, it may make sense to improve the UI for autocompleting users, or for hinting about the use of the field, to make it easier for sysadmins. But that shouldn't break the more common case of handling sites that just think they're Too Special or Too Important to allow saving login information.

[alexssung]

Ad blockers are extensions to browsers.

And I would actually argue that blocking before closing the tab should be a decision up to the developer, not the browser.