[whatwg] Forced subtitles

[Eric Carlson]

 In working with real-world content with in-band subtitle tracks, I have realized that the spec doesn't accommodate "forced" subtitles. Forced subtitles are used when a video has dialog or text in a language that is different from the main language. For example in the Lord of the Rings, dialog in Elvish is subtitled so those of us that don't speak Elvish can understand. 

 This is only an issue for users that do not already have subtitles/captions enabled, because standard caption/subitle tracks are expected to mix the translations into the other captions in the track. In other words, if I enable an English caption track I will get English captions for the dialog spoken in English and the dialog spoken in Elvish. However, users that do not typically have subtitles enabled also need to have the Elvish dialog translated so subtitle providers typically provide a second subtitle track with *only* the forced subtitles. 

  UAs are expected to automatically enable a forced-only subtitle track when no other caption/subtitle track is visible and there is a forced-only track in the same language of the primary audio track. This means that when I watch a version of LOTR that has been dubbed into French and I do not have a subtitle or caption track enabled, the UA will automatically show French forced subtitles if they are available.

 Because forced subtitles are meant to be enabled automatically by the UA, it is essential that the UA is able to differentiate between "normal" and "forced" subtitles. It is also important because forced subtitles are not typically listed in the caption menu, again because the captions in them are also in the "normal" subtitles/captions. 

 I therefore propose that we add a new @kind value for forced subtitles. "Forced" is a widely used term in the industry, so I think "forced" is the appropriate value.

eric