https://www.bleepingcomputer.com/news/security/css-is-so-overpowered-it-can-deanonymize-facebook-users/

Some of the recent additions to the Cascading Style Sheets (CSS) web standard are so powerful that a security researcher has abused them to deanonymize visitors to a demo site and reveal their Facebook usernames, avatars, and if they liked a particular web page of Facebook.

Information leaked via this attack could aid some advertisers link IP addresses or advertising profiles to real-life persons, posing a serious threat to a user's online privacy.