WEBVTT

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Fun fact: VTT cue identifiers
are completely optional.

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This file has no IDs at all.
Just timestamps and text.

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Some editors add them automatically.
Some don't bother.

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Both versions are perfectly valid
according to the W3C spec.

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ConvertICO's VTT viewer handles
both styles without any issues.

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When converting VTT to SRT,
sequential numbers are added.

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Because SRT requires them.
Formats have opinions, apparently.