Convert HtmlCaptions to NonHtmlCaptions when captionContainer is not Specified

[xxoo]

Is your feature request related to a problem? Please describe. iOS cannot display subtitles in fullscreen when using a custom container to render them. Cause iOS browsers do not support the standard fullscreen API. This may not have been a problem in the past, as MSE support was not introduced before iOS 17.1. But it has changed nowadays.

Describe the solution you'd like Since VTT subtitles do not require an additional container. Converting other subtitle formats to VTT should be able to solve this problem.

[nigelmegitt]

Converting to VTT subtitles is likely to be lossy, in that it may well change the presentation compared to the authorial intent, because other formats are more expressive. I do not think this is a great solution, but it could be acceptable to some folk.

I would very much not like dash.js to do this with BBC subtitles unilaterally, i.e. without some programmatical control to disable the behaviour.

[dsilhavy]

Converting to VTT subtitles is likely to be lossy, in that it may well change the presentation compared to the authorial intent, because other formats are more expressive. I do not think this is a great solution, but it could be acceptable to some folk.

I would very much not like dash.js to do this with BBC subtitles unilaterally, i.e. without some programmatical control to disable the behaviour.

I second that. We are also preferring to render WebVTT subtitles in the future ourselves instead of relying on the native implementation. We keep this issue open to gather additional feedback and experience.

[xxoo]

I believe this solution is likely to have insurmountable limitations. Otherwise there would be no point in using a separate subtitle container. But this proposal is made as a remedy or fallback option. Cause subtitles with some styles missing may still be better than no subtitle at all.