iBooks Author support for

Hi, I'm trying to add captions to a video clip in iBooks Author, and there seems to be very limited formatting for the captions - i.e., controlling how and where they appear on the screen, not how to create the webvtt file. Can anyone please tell me what the current version of iBooks Author is supposed to be capable of in this regard today, and/or direct me to documentation for the currently implemented caption styling features?

For context:

The originating html is <track src="./Videos/MVI_2107.vtt" kind="captions" srclang="en" label="English" default/>

The widget html works in Safari and Firefox. With iBooks Author I can insert the widget into my draft, the Default.png image displays, but neither the video nor the captions display. If I preview to iBooks on my Mac or to my iPhone, both the video and the captions display.

Thanks