http://www.huffingtonpost.com/2010/12/03/youporn-perez-hilton-expl_n_791479.html
Here is the link to the pdf.
http://cseweb.ucsd.edu/users/lerner/papers/ccs10-jsc.pdf
Apparently, these sites exploit the history visited color via DOM of webpages. So, when you visit their website, they have hidden links to sites such as nytimes, facebook, etc...and then via javascript determine the color and thus if you've visited them or not.
Here is the link to the pdf.
http://cseweb.ucsd.edu/users/lerner/papers/ccs10-jsc.pdf
Apparently, these sites exploit the history visited color via DOM of webpages. So, when you visit their website, they have hidden links to sites such as nytimes, facebook, etc...and then via javascript determine the color and thus if you've visited them or not.