This page hooks up mouseenter and mouse leave events to the document body, and to the red box below.

For both the document body and the red box, capture mode is set to true when calling addEventListener (not done on older IE of course).

When you move your mouse over the red box, you should get a mouseenter event for that box, as well as event for the document body because mouseenter/mouseleave events ARE NOT cancellable according to the spec

Instead in Firefox (tested in version 23 and 24) you only get an event firing on the document body, b/c Firefox is allowing you to cancel / stop propagation down to the box.