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Unless I'm mistaken, in the table of section 4.8.9.12 Event summary [1], the progress event should use the interface ProgressEvent, and not simply Event. In the table of section 5.6.1.1 Event summary [2] properly lists the ProgressEvent interface. [1] http://dev.w3.org/html5/spec/Overview.html#mediaevents [2] http://dev.w3.org/html5/spec/Overview.html#appcacheevents
We moved away from using ProgressEvent for media elements because media elements have their own way of tracking progress already which is more suitable. (They also never complete loading.)