Firefox developers recently responded my bug report that I previously mentioned, about HTML5 media events mistakenly being cancellable, even though this contradicts the specification and has no effect. The conclusion is that

this is just a case of the spec changing and nobody noticing.

The specification was updated in April 2009 to include the behaviour I discussed. Firefox, WebKit and Opera all implemented the events before that, and didn’t entirely update their behaviour to reflect the new changes. A patch for Firefox has now been uploaded and tickets have been filed against WebKit and Opera.

Ironically, Internet Explorer 9 and 10 apparently do conform to the spec — probably because they didn’t implement any HTML5 media until years after the changes were made.