Using Progress Observers
  introduction to HTML_Progress observer mechanism
 
 Table of Contents
 Introduction
The HTML_Progress_Observer class provides an implementation of the observer pattern.
   In the content of the HTML_Progress package, they provide a mechanism which you can
   examine each important event as it is happened. This allows the implementation of
   special behavior based on the value of the progress element.
 Learning part
If you want to know more and learn how to use the observer mechanism
  of HTML_Progress 1.x, read article 
  HTML_Progress_Observer.