Declared WebVTT subset
Validate a WebVTT subtitle file
Check the WEBVTT header, cue blocks, supported settings, adjacent timing, and the same visible readability profile used by the full checker.
What does a WebVTT validator check?
A WebVTT validator checks the WEBVTT header, cue timing syntax, cue blocks, identifiers, and supported cue settings. Subtitle Preflight parses a declared WebVTT subset in your browser, keeps parser diagnostics separate from profile findings, and does not claim complete WebVTT conformance.
Check a supplied subtitle file
Supported: .srt, .vtt, and .webvtt text files. Current browser-local limits: 5 MiB and 10,000 cues.
Or drag and drop one supported subtitle file here.
Privacy: Analyzed in this browser; subtitle contents are not sent to our application server in the default workflow.
Findings
No subtitle loaded
Choose a file, paste subtitle text, or load a synthetic sample to see cue-level findings here. Export becomes available after analysis succeeds.
The checker does not compare subtitles with audio or video and cannot verify accuracy, completeness, speaker identification, meaningful sounds, visual placement, accessibility or legal conformance, or client/platform acceptance.
Minimal WebVTT example
WEBVTT
cue-1
00:00:01.000 --> 00:00:03.000 align:center
Synthetic WebVTT cue.WebVTT begins with the WEBVTT header and uses dot milliseconds. A cue identifier is optional. The declared parser supports common line, position, size, align, and vertical settings; STYLE, REGION, and unknown settings receive diagnostics.
WebVTT structure and cue checks
The implementation reads a WEBVTT header, optional cue identifiers, dot-millisecond cue timing, cue text, NOTE blocks, and common line, position, size, align, and vertical settings.
Timing and readability checks after parsing
Parsed cues receive deterministic overlap, gap, duration, CPS, WPM, line, empty-text, and duplicate-text checks. Parser errors remain separate from profile warnings.
WebVTT source and implementation boundary
The declared subset is informed by the W3C Candidate Recommendation Draft, WebVTT: The Web Video Text Tracks Format, published 20 May 2026. Subtitle Preflight parser version 1.0.0 supports only the structures listed on this page. It is not a complete conformance checker, is not W3C-approved, and may report unsupported structures separately.
Read the W3C WebVTT Candidate Recommendation Draft dated 20 May 2026
Review the declared WebVTT subset and limitations
Export WebVTT findings for review
Fresh findings can be downloaded as CSV or a self-contained HTML report generated in the browser.
Check a WebVTT file before delivery
Run the declared-subset parser, review cue-level timing and readability findings, and export a local report when the result is fresh.
Frequently asked questions
Are .vtt and .webvtt accepted?
Yes. Both extensions are accepted, and pasted text can be detected from its WEBVTT header.
Is this a W3C endorsement?
No. The tool cites the source that informed its documented subset and does not imply endorsement or complete specification conformance.
Are cue settings auto-fixed?
No. Unsupported settings are reported non-destructively.