Implementing "this page is asking you to confirm that you want to leave"
<p>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 filling in the forms, I can only assume that it's working on a dynamic page. Which technology is used to implement this functionality? How can I implement it myself on a simple hello-world page?</p>
