Geoff: yes, these days the file is just a text file that contains captions that have been formatted using one of two or three caption formats: webttt, ttml and srt, all browsers support that. Any browser based tool will rely on the browser to interpret it.
Joshua: What’s the file format?
Geoff: I believe .ttt is the preferred file for webttt is not xml which is why people like it. It’s a simpler construction. The problem with it and its support, it allows for styling of text to look big small, red, green, and etcetera. Not all browsers support all the styling and placement attributes that are available in the current recommendation. It’s not a final one yet. But if you’re just talking about a very straightforward presentation of captions it should work fine in any system.
Joshua: This a problem in the EPUB spec or is it only a reading system issue, because it seems to me to be both. If it’s not required of the core media type then support for it isn’t immediately required. If you look at things like font, there are specific fonts listed in the spec and listed as core. So it might make sense to go that way. So when a developer looks at the core specs they know they have to support it to.
Geoff: When you’re looking at it that way, then yes, I agree with you. The actual caption file is a separate entity from everything else. So the format of the file itself is independent, but has to be supported as a core format so reading systems know what to do with it. I would recommend that Webttt and ttml be used.
Charles: Why wouldn’t you just embed the closed captioning into the video? Like combine it in there so you would have the video itself that has the closed captioning as part of it.
Geoff: You can do that with a .ftt format. The approach works for QuickTime media, but the reason not too many people use that is because it involves extra rendering of the video. It isn’t as easy to edit. So if you need to make a correction to a caption file that has been created with WebTTT or ttml you just open the file and correct the test. With embedded ones you have to open the movie and then re-render it.
Joshua: you have to have that format specifically mentioned in the core media type. There isn’t a lot of support for anything other than standard n14 formatting.
Geoff: There isn’t a lot of need to embed caption data and the tools to do it, but these days it’s more flexible to use sidecar format.
There’s no change since EPUB 3. We recommend doing the same thing that HTML 5 (now 5.1) recommends. See the video element  and the track element , specifically the note about accessibility. I am a little confused about why VTT needs to be a core media type.
What’s missing from the spec?
Also note that files referenced from the track element have been exempt from Core Media Type fallback requirements since 3.0, so I don't see that we need to change anything on the EPUB side. We made the exemption for this reason.