Works for me. You first need to click the "try it yourself on this page" and then move the mouse around and try hovering over various parts of the page. The bla

Works for me. You first need to click the "try it yourself on this page" and then move the mouse around and try hovering over various parts of the page. The black overlay box follows your mouse cursor and while it is initially empty, once you start moving the pointer, the rules start appearing in it.