Consider adding accessibility considerations for authors

[tidoust]

WebCodecs is a low-level API, which should not have direct accessibility implications for implementations.

WebCodecs makes it easier for applications to implement their own media player. Built-in media players come with accessibility related features, including the ability to slow or pause playback, support for captions, or switching to an audio description stream. The spec already has a Best Practices for Authors Using WebCodecs section. It could perhaps be completed to include accessibility considerations ("with great powers come great responsibility").

[aboba]

In practice, accessibility is not just a media player concern - it requires support in container formats (e.g. CMAF includes support for captioning), codecs such as Real Time Text, signaling/language negotiation, etc. However, these areas are outside the scope of WebCodecs.

[chrisn]

See also the WebCodecs accessibility self-review: #856