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The January 3rd, 2013 version of the spec says: A WebVTT file <http://dev.w3.org/html5/webvtt/#webvtt-file> whose cues all have an end time offset x greater than or equal to the end time offsets of all the cues whose start time offsets are less than x is said to be a WebVTT file using only nested cues. An example of such a file is then given, starting as follows: WEBVTT 00:00.000 --> 01:24.000 Introduction 00:00.000 --> 00:44.000 Topics If you look at the second cue (x = 00:44.000), then the first cue is a cue whose start time is less than x (it's 00:00.000), however x is not greater than or equal to the end time offset of the first cue (which is 01:24.000). I think that a correct definition would be: A WebVTT file which, given any two cues with start and end time offsets (x1, y1) and (x2, y2) respectively, has either (x1 <= x2 and y1 >= y2) or (x1 >= x2 and y1 <= y2), is said to be a WebVTT file using only nested cues. (I'm not suggesting that this be the exact wording.)
Fixed in https://dvcs.w3.org/hg/text-tracks/rev/10b7ff6505ca and https://dvcs.w3.org/hg/text-tracks/rev/19a5da54cec6
Follow-up bug 25737