http://dvcs.w3.org/hg/html-media/raw-file/tip/media-source/media-source.html#mediasource-seeking
http://dev.w3.org/html5/spec/media-elements.html#dom-media-seek

MSE hooks in to the HTMLMediaElement by saying "The media element seeking algorithm starts and has reached the stage where it is about to fire the seeking event." HTML's step 8 says "Queue a task to fire a simple event named seeking at the element." MSE doesn't say if its steps should be run before the event is queued or before it is actually fired, but the intention is clearly to insert steps before step 8.

Since MSE replaces the network layer, it would make more sense to hook in to step 9: "Set the current playback position to the given new playback position." This is the step of the HTMLMediaElement seek algorithm where the media framework underlying HTMLMediaElement will be notified and begin to fetch the required data, so it is where MSE would come into play.

Suggestion: Specify that the steps of MSE run as part of step 9, or between step 9 and 10. This would make more sense in Opera's layering, and the only observable difference is the order of the seeking and sourceopen events.

Finally, note that MSE points to #dom-media-seeking instead of #dom-media-seek, almost certainly a typo.