There is NO explanatory text anywhere.. which HTML source have you seen?
The tutorial is really just title, code area and results :-)
This is a "learn by example" tutorial, aimed at who already knows a little javascript.
I started this tutorial with zero prior knowledge of Javascript, but I have still learned a lot from it. I am now 2/3 of the way through. The challenge of guessing what will work helps to keep me engaged. The extra time that I need to spend on a slide, compared to someone who already knows a little Javascript, is probably paid back in extra retention. I love this tutorial and am excited about adding a little Javascript to my projects.
I just did a view source on one of the pages to see if I was missing something, and it was full of what looked like explanations: loads of text in HTML definition lists, which seemed a bit unusual, but I figured that was something to do with how the material was meant to be structured.