This page teaches how event bubbling works. Using the panel to the left, select the events you want tracked. Every time one of the specfied events occurs in this frame, it will appear in the text box. Remember that events bubble, so a large number of events may be generated (try mousemove for an example).
Looking behind this page you will find no source code. This is because all the hard work is being done by the other frame. This allows you to easily experiment with any sample document.