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The "Sourcing out-of-band text tracks" section doesn't specify a task source for the various tasks it queues up.
The problem is specifically with the tasks in the "start the track processing model" algorithm, since there might not be a media element around when they get queued.
Checked in as WHATWG revision r8574. Check-in comment: try to fix ambiguous task queuing (which source should be used), for <track> http://html5.org/tools/web-apps-tracker?from=8573&to=8574