You can do a hack with ETags - that was the first thing I came up with after 30 seconds of brainstorming how you would do it, and it looks like sites are alread

You can do a hack with ETags - that was the first thing I came up with after 30 seconds of brainstorming how you would do it, and it looks like sites are already using them for that purpose. http://en.wikipedia.org/wiki/HTTP_ETag#Tracking_using_ETags http://en.wikipedia.org/wiki/HTTP_ETag#Tracking_using_ETags