Wikipedia has a list of websites known to use this technique already: https://en.wikipedia.org/wiki/HTTP_ETag#Tracking_using_ETags
Looks like KISSmetrics are getting sued with a class action lawsuit over using this technique.
Looks like KISSmetrics are getting sued with a class action lawsuit over using this technique.
ETags are one of the biggest disappointments that way.
I wonder if there's a potential to fix this by suggesting a deterministic version of ETags, that's based off of information the User Agent can see. Like the Vary information, response headers and payload.