The HTMLTrackElement.src property reflects the value of the <track> element's src attribute, which indicates the URL of the text track's data.

HTML HTML DOM HTMLTrackElement Property Reference src Web WebVTT

WebVTT is a format for displaying timed text tracks (e.g. subtitles or captions) with the <track> element. The primary purpose of WebVTT files is to add text overlays to a <video>. WebVTT is a text based format, which must be encoded in UTF-8 format. Where you can use spaces you can also use tabs.

API captions Intermediate NeedsUpdate Reference subtitles text tracks Web Video Text Tracks WebVTT