Wait, how is he doing this? Wish there was a tiny bit more explanation in the post...
Most of the Asteroids in the game are links - they use those same CSS rules to color themselves based on whether you've visited them or not.
Script on the page can't tell what color the asteroids are, but it can tell what asteroids you've clicked on. By making the visited-link color the same as the background color of the webpage, we get the user to give away the color of the links, because they'd only be clicking on the links that they can see - the ones with a color different to the background.
Btw, why do they want to do this?
If you launch a ton of these types of games, you could build a pretty decent database of user's browsing histories. If you are using news sites as your sites of interest, you could use that browsing history to build up demographic data about each of the people who play your game (demographics for large sites like CNN are pretty well known).
There are certainly other reasons, but this is the first one that came to mind.
a:visited { background-color:pink;}
Because you're clicking the border and not the element which is 0x0 you're not redirected but he knows the mouse position and from there can figure out which link you clicked on.