I suppose it uses CSS to steal your browsing history.
For those that have not seen that post before: simple proof of concept[1] and a game[2] that will try to steal your browsing history. Obvious warning: visiting those sites might lead to your browsing history being stolen.
[1]http://tinsnail.neocities.org/ ( http://lcamtuf.blogspot.de/2013/05/some-harmless-old-fashion...