text/vtt

The web's subtitle format, and the only one the HTML track element officially speaks: WebVTT grew out of the fan-subtitling SRT format (its first name was WebSRT) and W3C-ified it with cue positioning, styling hooks, and mandatory UTF-8. Send it as text/vtt with CORS headers in order — subtitles are fetched cross-origin from CDNs constantly, and a missing Access-Control-Allow-Origin is the classic reason captions silently fail. Every file must begin with the ASCII string WEBVTT on its first line, optionally after a byte-order mark, which gives this text format an honest sniffable signature.

.vtt only; .srt files are the ancestor format and need conversion, not relabeling.

Defined by WebVTT: The Web Video Text Tracks Format (W3C) · IANA registration. Registry facts from the IANA media-types registry via mime-db.