Hey there! I'm glad you like it. :)
The code that generates the dungeons is running on a server. When you visit the page or click the "give me another dungeon" link, all it does is request a new dungeon from the server. The server then does the random generation, lays out the diagram and sends it back to this page. That's why there's so little code -- it's all on the server.
Wow, realy cool! Thank you very much for answering the question. I did not know that different cool things can be done on the server.
Also, I would like to write my guesses on how this works, I look through the developer console, and I see that the link "Give me a new dungeon", restarts the page using JavaScript. I think that after the page is restarted, a new dungeon is sent from the server.
