How to insert an unstoppable popup in your web page?

How does an unstoppable popup work?

An unstoppable popup is in fact a <div> </ div> tag containing HTML code that, when interpreted by the web browser, looks like a window that floats over the normal web page. Please see this example http://www.1stoppopup.com/UnstoppableExemple.htm

If you look at the source code of the above mentioned page [...]