History Sniffing: How YouPorn Checks What Other Porn Sites Youve Visited and Ad Networks Test The Quality of Their Data
www.forbes.com/sites/kashmirhill/2010/11/30/history-sniffing-how-youporn-checks-what-other-porn-sites-youve-visited-and-ad-networks-test-the-quality-of-their-data
From the page
Saved with the link. Notes from 2007 to 2016 are mostly the page's own words, so this is probably an excerpt.
Checking the browser state of hyperlinks to map out where you have been.