This is frustrating because this isn't a difficult UX problem to mitigate. Opera 12 for example had excellent distinction between in-page and browser-powered UI

This is frustrating because this isn't a difficult UX problem to mitigate. Opera 12 for example had excellent distinction between in-page and browser-powered UI prompts. It will always be possible to socially engineer some people, but making it a bit more obvious isn't difficult. Firefox at least has very slightly more distinct pop-downs than Chrome's.