Separate the text track from the picture
Subtitles or captions can be delivered as a distinct timed-text resource rather than being permanently drawn into the video. Its format and timing matter to the player independently of the video codec. A picture appearing without the expected text does not automatically mean the selected audio language is wrong. First identify whether the text is a selectable track, an external file or part of the image itself.
Recognise two different descriptions
WebVTT describes cues and their timing for text tracks, while TTML uses an XML-based timed-text model. They are not merely two extensions for identical content. At this review, the current W3C WebVTT publication is a Candidate Recommendation Draft; TTML2 is a W3C Recommendation. These document statuses should be described accurately rather than turning every referenced technical document into a final universal compatibility promise.
Check the actual player route
Use the format and track supplied for the legitimate viewing arrangement. A player may support one text workflow without supporting every styling feature or another format. Renaming a subtitle file does not translate its structure. When reporting a problem, include the player, platform, text format and whether the issue concerns visibility, timing or rendering. Keep the exact programme edition in the record where it affects alignment.
Keep accessibility expectations specific
A language subtitle, closed-caption track and audio-description track address different needs. A compatible text parser does not create information absent from the programme. Test the required option on the device you intend to use and describe which controls are available. This is more useful than assuming that one subtitle logo guarantees every language, caption detail or styling preference across all programmes and applications.
Questions, answered
Can I convert TTML into WebVTT just by changing the extension?
No. A different filename does not translate the timed-text structure or presentation model. Use a supported track or an appropriate authorised conversion workflow rather than treating extensions as interchangeable.