A set of tests for webkit-prefixed animation event handlers/listeners were added in #19099:

• wpt/dom/events/webkit-animation-end-event.html
• wpt/dom/events/webkit-animation-iteration-event.html
• wpt/dom/events/webkit-animation-start-event.html
• wpt/dom/events/webkit-transition-end-event.html

However as per #19099 (comment), these tests only test IDL attribute handlers and event listeners, but not content attribute handlers (i.e. <div onwebkitanimationend='foo()'></div>). We should expand the tests to cover content attribute handlers too. This will require some experimentation as to how content attribute handlers interact with IDL attribute handlers when both are set, as well as the easier task of adding a test for case sensitivity.

Note that for case-sensitivity, there is a already a positive-match test for the event casing in html/webappapis/scripting/events/event-handler-all-global-events.html ('the content attribute must execute when an event is dispatched'), but not a negative-match test making sure that an incorrect casing doesn't cause the content attribute to fire.