How do word timestamps and speaker labels map into WebVTT (cue timing, in-cue word timestamps, voice spans, line breaks), and what makes a file invalid?

Problem: turning an aligned, diarized word table into subtitle files that parse. Conditions: WebVTT per the W3C Candidate Recommendation Draft of 20 May 2026. Method: one cue block per readable chunk; cue start must be greater than or equal to every earlier cue's start and end must be greater than start; cues may overlap, which suits simultaneous speakers; speaker names go in voice spans (<v Name>), which need no closing tag when they cover the whole cue; word-level timing inside a cue uses cue timestamps, each greater than any previous one in the cue, greater than the cue start and less than the cue end; the spec's own guidance is to write cues on one line except where a break is definitely needed, since user agents insert wrapping breaks. WhisperX's CLI can also write word-highlighted SRT. Failure modes: Inference: word times from an aligner that equal the cue start or end, or that decrease, produce invalid timestamps; clamp to the open interval and drop duplicates; a mislabeled speaker puts the wrong name inside the voice span. Dubbing and lip-sync use: Inference: the same word spans give per-word durations that can be compared with a dub script's syllable timing; this source says nothing about dubbing. Limitations: line-length and reading-speed limits are not in WebVTT (see prior Netflix/BBC/EBU findings). Version/date: W3C CRD 20 May 2026 as fetched 2026-10-24. Curator qualifications: All cited documentation excerpts located on the live pages by curator GET; no video sources. Limitations: Validity consequences and dubbing use are inference from the spec's ordering rules. Evidence status: statements located in the cited sources by a correlated desk review (Claude); not executed; not independently reviewed.

{
  "citations": [
    {
      "url": "https://www.w3.org/TR/webvtt1",
      "note": "cited at https://www.w3.org/TR/webvtt1/; 4.2 WebVTT cue timings; WebVTT cue timestamp; 1.x voice example; 1.2 Caption cues with multiple lines; Usage; researcher reasoning"
    },
    {
      "url": "https://raw.githubusercontent.com/m-bain/whisperX/main/README.md",
      "note": "4.2 WebVTT cue timings; WebVTT cue timestamp; 1.x voice example; 1.2 Caption cues with multiple lines; Usage; researcher reasoning"
    },
    {
      "url": "https://www.w3.org/TR/webvtt1/",
      "note": "observed URL (dated/alias form of the cited catalogue source; canonical identity retained)"
    }
  ]
}