Validate internal WebVTT timestamps within a cue

The useful distinctions

• WebVTT permits timestamp markup inside cue text.
• Each internal timestamp must be later than the cue start and earlier than its end.
• Internal timestamps must increase within the cue.

Expand the cue into activation boundaries

The cue occupies seconds ten through fourteen. Its first words are available from the cue start, the next phrase is marked at eleven seconds, and the final word at twelve and a half. A review worksheet therefore has boundaries at ten, eleven, twelve and a half, and fourteen. The first internal boundary is not a new cue start, and the two internal markers do not divide the file into three independent cue blocks.

Test the boundaries directly

Convert all times to milliseconds and check a strict ordering: ten thousand, eleven thousand, twelve thousand five hundred, fourteen thousand. This catches a marker accidentally copied from a different cue even when its timestamp is otherwise grammatically well formed. A marker exactly at the cue end also fails the stated containment rule; it is not merely a zero-length final word. Internal timing can support progressive presentation, but the file text alone does not promise a specific animation or highlight design. Keep any rendering expectation separate from the timing validation. If an editor moves the enclosing cue, review the internal values as well rather than changing only the outer timing line and leaving the payload's absolute markers behind.

Worked example · synthetic

The required ordering is 10.000 < 11.000 < 12.500 < 14.000 seconds. The internal markers are valid within the enclosing interval; a hypothetical 14.000 marker would fail the strict upper bound.

Synthetic text example; no network operation has been performed.

WEBVTT

00:00:10.000 --> 00:00:14.000
Now <00:00:11.000>fold the <00:00:12.500>corner.

Check the interpretation

• Convert every boundary to the same unit.
• Require strictly increasing internal markers.
• Recheck internal markers after moving an outer cue.