HTML5 refuses SRT without a header

The <track> element accepts WebVTT and nothing else. A file named captions.srt looks right, but when you reference it with <track kind="captions" src="captions.srt">, the player simply shows no captions and reports no obvious failure. The requirement is blunt: the file must begin with the literal string WEBVTT on its first line. An SRT file is otherwise structurally close — the same blocks, the same timings, the same text — but without that header it is not WebVTT and the browser will not load it. This is the single most common reason captions "silently do nothing" on a web page.

The decimal separator is not optional

SRT writes milliseconds after a comma: 00:01:23,456. WebVTT uses a dot: 00:01:23.456. A spec-strict parser treats the comma as an error. This tool is deliberately lenient on input, accepting either separator because real-world SRT files vary, but its output is always strict WebVTT with dots.

One more thing has to be right outside the file itself: serve it with the MIME type text/vtt. A server that sends text/plain — which many do for an unrecognised extension — produces exactly the same silent nothing as a missing header, and it is a much harder problem to spot because the file itself is perfectly valid.

Cue numbers vanish in the conversion

SRT conventionally puts a number before each block (1, 2, 3). WebVTT does not require numbering, so this tool does not emit any — the timing and text carry over untouched and only the numbers are dropped. WebVTT's own optional cue identifiers are not generated either, so if you need to reference a specific caption from your site's JavaScript, that labelling has to be added after conversion.

Worth knowing in the other direction: the parser treats the SRT number as optional, so a file that has lost its numbering still converts fine.

Timing accuracy is preserved as integers

Times are parsed to whole milliseconds and re-serialised from those integers, so nothing drifts no matter how large the file or how many conversions you chain. Blocks whose timestamps cannot be parsed are skipped rather than aborting the run, and an amber warning names them by block number — up to five, then a count of the rest. Only a file with no parseable cues at all is rejected outright. Try the VTT to SRT Converter to reverse the process.