A new method has been discovered that allows a web site to track the mouse movements of their visitors by using only HTML and CSS, which can bypass tracking protection. Security researcher Davy Wybiral recently demonstrated on Twitter how web sites can use HTML and. CSS to watch mouse movements occurring in one browser window from within another browser window. A user in another browser can then use the /watch URL to watch in in real time what grid box is currently being hovered over. This technique can be used for a variety of reasons including determining hot spots on a site or for user interface studies.
Source: https://www.bleepingcomputer.com/news/security/researcher-finds-css-only-method-to-track-mouse-movements/