On april 8th 2013 I delivered a free lecture about HTML. The lecture took place in HIT. You can download the slides at http://www.lifemichael.com/presentations/htmljumpstart.pdf. You can find (within the slides) links for downloading the code samples I was using during the lecture.
The Visitor Design Pattern
The visitor design pattern allows us to add operations to objects that already exist without modifying their classes and without extending them.