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The parsing rules currently allow cues with negative duration. Since these cues do not represent proper cues and handling them the same as positive duration cues might generate an unexpected behavior, it would be better to drop them from the beginning, when the file is parsed. Zero cues duration, however, should not be dropped, as they might represent instantaneous events for metadata tracks.
http://lists.w3.org/Archives/Public/public-texttracks/2012Apr/0107.html