I assume that it's simply setting styles on links using the ":visited" pseudo-selector, and then either:

1. Using JavaScript to get the rendered style, and thus work out whether you've visited a particular URL, or

2. Setting background images on links which only apply when visited, and checking whether your browser retrieved those images.

It's a rather old and well-known trick.

Ok. Since I couldn't access the site, I didn't even know what it was supposed to be doing... other than 'an old and well-known trick' and something to do with browser history.