ActiveSupport.Element is a simple DOM manipulation library that does not modify the built in Element object. All ActiveSupport.Element methods take an Element object (and not a string) as their first argument. ActiveSupport.Element is available inside ActiveView classes as the second argument:

var MyClass = ActiveView.create(function(builder,dom){
    var link = builder.a({href:'#'},'Text');
    dom.addClassName(link,'active');
    dom.getWidth(link);
    return builder.div(link);
});

The implementation of event obeserver's differs from Prototype's since it does not modify the Element object. Your observer receives three arguments, the Event object, a function that will stop the event when called, and a function that will unregister the observer.

var dom = ActiveSupport.Element;
dom.observe(link,'click',function(event,stop,unregister){
    //do stuff
    stop();
});

ActiveSupport.Element also supports the a similar event to Prototype's dom:ready:

dom.observe(document,'ready',function(){
    //...
});