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http://dvcs.w3.org/hg/html-media/raw-file/tip/media-source/media-source.html#source-buffer-duration-updates "If the current playback position is beyond the new duration, then update HTMLMediaElement.currentTime to the new duration and run the seeking algorithm." http://www.whatwg.org/specs/web-apps/current-work/multipage/the-video-element.html#durationChange "If the duration is changed such that the current playback position ends up being greater than the time of the end of the media resource, then the user agent must also seek the to the time of the end of the media resource." It would be possible to just remove the redundant requirement from MSE, but referencing HTML in a note for clarity is also fine. (Also, the quoted text looks a lot like an implementation requirement in what is actually a non-normative overview.)
I believe this was addressed in the changes for https://www.w3.org/Bugs/Public/show_bug.cgi?id=18576 . The referenced section and text aren't in the spec anymore and the duration change algorithm references the appropriate text in the HTML5 spec.