Jeremiah Grossman came up with a really clever way of using Javascript to find what pages you’ve visited recently. It goes like this: when a link has been visited, you can use the :visited pseudo-class to style it. Using Javascript, you can walk through all of the links on your page and grab their styles; comparing a link’s style to the style given to visited links in the CSS will tell you whether or not a site has been visited. Seems innocent enough, until you realize that this can be applied to a large list of links. A site will know whether or not have been to any of the sites on their list. Combine that with a little Ajax, a database and a cookie, and you can track where your return users have been over time.
