Add comment describing what test_compute_data_with_start_struct.html does.  No bug.  No review.

(Followup to IRC discussion with nrc, who hit a failure in this test.)
Bug 959832 patch 1: Fix tests in test_eventctors.html that weren't testing what was intended.  r=smaug
Bug 959832 patch 2: Transition events and animation events are not cancelable.  r=smaug