Fortunately, for those of us who think this crosses the line on privacy, breaking every test case on the site only takes one line in a user.css file:

a:visited {color:blue !important}

And yes, that line has already been loaded into Safari on my system, and it's going to stay there. Although I admire the cleverness of the programming Niall used, I don't appreciate exposing my browsing history to random websites through unexpected methods. Niall's approach breaks the principle of least surprise, and it plays fast and loose with what most users would consider private data - I can't see how implementing this technique in the real world could be seen as a good idea. Just because you can doesn't mean that you should.