“Despite this strict separation, the presentation and behavior layers need instructions from the structural layer. They must know where to add that nice touch of style, when to initiate that smooth bit of behavior. They need triggers.”
 
Now that you’ve separated your Web site’s structure (XHTML) from its
presentation (CSS), wouldn’t it be great to similarly abstract the
behavioral layer (JavaScript) from the others? Peter-Paul Koch shows
how to use JavaScript Triggers to do just that.