The checks every file goes through

Each cue must be: a sequence number, a timing line, one or more lines of text, then a blank line. Blocks that don't follow this are reported as malformed with their line number.

1
00:00:01,000 --> 00:00:03,500
Welcome to SNUBBIX.

Timestamps must be HH:MM:SS,mmm — two-digit minutes and seconds below 60, a comma before three-digit milliseconds, separated by " --> ".

00:00:03.700  ✗  full stop instead of comma
00:00:03,700  ✓

Numbers must start at 1 and increase by one. Missing, duplicated or out-of-sequence numbers are flagged and corrected on delivery.

Every end time must be later than its start time. Zero-length and negative cues are errors.

Cues must be in chronological order of start time. A cue that starts before the previous one is an error.

A cue starting before the previous cue ends is flagged. Some overlaps are intentional (e.g. two speakers); a reviewer decides against your rules.

Default rules (adjustable per project): ≤ 42 characters per line, ≤ 2 lines per cue, ≤ 20 characters per second, 0.833–7 s on screen. Flags are reviewed in context.

Files must decode as UTF-8 (UTF-16 is flagged), contain no binary data, and use consistent line endings. Byte-order marks are noted.

Before delivery a person reviews the validation results, file names and the delivery package against the quotation.

Where included in your quotation, timing is spot-checked against the source media. This is the only check that compares subtitles to what is actually spoken.