The CSS Animations Level 1 extends [1] the GlobalEventsHandler Interface defined in the HTML spec [2], declaring 4 new event handlers: onanimationstart, onanimationiteration, onanimationend and onanimationcancel. Only the 'onanimationcancel' event handler has been missing from the GobalEventsHandler IDL. [1]https://drafts.csswg.org/css-animations/#interface-globaleventhandlers [2]https://html.spec.whatwg.org/multipage/webappapis.html#globaleventhandlers
The 'onanimationcancel" event handler should be exposed through the GlobalEventHandlers interface so that web applications can detect when the animation stops running in a way that does not fire an 'animationend' event, such as a change in the 'animation-name' that removes the animation, or the animating element or one of its ancestors becoming display:none.
Docs: https://developer.mozilla.org/en-US/docs/Web/API/Element/animationcancel_event
Samples: https://developer.mozilla.org/en-US/docs/Web/API/Element/animationcancel_event#examples