Browser user agent. Mine is unique for example. With sufficiently vast DBs they can match you or anyone with anything.
Also some 3rd party software may request to spy on you in the browser, e.g. Red Shell so called "analytics" used by many legitimate big video games on PC. It will spy on you in the browser and you agree to it in a kilometer long user agreement they shove in your face upon first launch.
Also some 3rd party software may request to spy on you in the browser, e.g. Red Shell so called "analytics" used by many legitimate big video games on PC. It will spy on you in the browser and you agree to it in a kilometer long user agreement they shove in your face upon first launch.