The kind of the text track, represented by a value from the following list:
'subtitles': The track provides subtitles, used to display subtitles in a video.
'captions': The track provides a translation of dialogue and sound effects (suitable for users with a hearing impairment).
'descriptions': The track provides a textual description of the video (suitable for users with a vision impairment).
'chapters': The track provides chapter titles (suitable for navigating the media resource).
'metadata': The track provides content used by scripts and is not visible for users.
- If an unrecognized value is provided, the player will interpret it as `'metadata'`.