2.2.2 [W3C-HTML51] Section 2.2.2 Dependencies

C0003: The WebVTT specification is a supported text track format for media resources

The specification states:

 2.2.2 Dependencies
     ...
     WebVTT
         Implementations may support WebVTT as a text track format for subtitles, 
         captions, chapter titles, metadata, etc, for media resources. [WEBVTT]

EdgeHTML Mode

The WebVTT specification is a supported text track format for media resources.