I want browsers to extract and expose embedded captions from video container files

I want browsers to detect, extract, and surface caption and subtitle tracks that are embedded inside video container files — such as MP4 and WebM — so that users can access them without authors having to duplicate them via the <track> element.

MP4 containers support embedded timed text tracks (CEA-608/708, TX3G, and others) and WebM containers can carry embedded subtitle streams. Many videos distributed on the web already carry captions baked into the container — especially content converted from broadcast or authored with professional video tools — yet browsers ignore this embedded metadata entirely. As a result, even fully captioned videos appear subtitle-free on the web unless the author separately extracts and re-provides those captions as WebVTT files referenced through <track>.

If browsers extracted embedded tracks automatically and presented them to users in the native caption UI, the result would be:

• Better accessibility by default: users would get captions on any video that was already captioned, with no extra work from the page author.
• Reduced duplication: authors who encode captions into their container would not need to maintain a separate WebVTT file alongside every video.
• Graceful degradation: explicit <track> elements could still override or supplement extracted tracks, preserving backward compatibility.

The Media Working Group has noted this is a valid request.