This program is running in a very simple Processing-style environment that works right on a web page. You can see the code and change it to fit your own desires. But what about this is really special to the Web? Most of this can be done (and is being done) on the desktop.
This page is published right in the middle of the Web, linked to from other pages; and it's easy to see the source, in the best tradition of HTML pages and Wikis. It's natural to put exposition like this along active elements. Furthermore, of course the exposition can contain linkstootherplaces.
Going beyond this, ideas for web-native programming environments might include some sort of linking inside programs, greater modularity a la HTML elements, closer integration with the page, and publishing or live sharing of data through web servers - and of course many others! For example, OpenCode uses the Web to help people share their web programs and so learn from one another.
(p.s. 30 June 2007: Of course I must have seen the similar TinyCode some time ago and forgotten about it! It is a small turtle-graphics system done on an HTML canvas.)