Implementing “this page is asking you to confirm that you want to leave”

Question

This is a warning that Firefox raises when I want to leave certain pages. Based on the pages I\'ve seen this on and that this warning appears when I try to close the page after

Answer 1

Well, you need to try to add some other things like form. But something simple is:

EDIT: Fixed HTML;

close this window and watch the warning box come up!