SRT and VTT differ in a few important ways.
SRT uses a comma before milliseconds and numbers each cue. WebVTT begins with WEBVTT, uses a full stop in timestamps and can carry positioning settings after the end time.
SRT numbering is removed when converting to VTT. Cues are numbered in order when converting to SRT. VTT settings without a direct SRT equivalent are not included in the SRT output.
Validation catches clear file errors.
The tool marks lines where it cannot read the timing, the end precedes the start, a cue has no text, or two cues overlap. A file with timing errors cannot be downloaded until the source is corrected.
This is a technical check, not language editing or a complete implementation of every WebVTT feature. Text, file names and timestamps are processed locally and are not sent to Idun.