Social History Javascript: What Social Networking Websites are Visited by Your Visitor

Today I came across a sneaky but very smartly implemented social history javascript. By embedding this javascript and making a few simple calls, you can find out if a visitor to your website or blog has visited a particular URL in the past. It does it in a very smart way (I'm not saying anything about whether the purpose of the script is right):

“By using a cute information leak introduced by CSS. The browser colors visited links differently than non-visited links. All you have to do is load up a whole bunch of URLs for the most popular social bookmarking sites in an iframe and see which of those links are purple and which are blue. It’s not perfect (which, from a privacy perspective, is at least a little comforting) but it does get you 80% of the way there. The best/worst part is that this information leak probably won’t be plugged because it’s a fundamental feature of the browser.”