I mentioned the difference between subtitles and captions at the outset, but the kind attribute can additionally take the following two values of note:

• descriptions — specifying this value indicates that the track contains a text description of the video. A descriptions track is designed to provide missing information to readers who can hear the audio but not see the video (which includes blind and low-vision readers, but also anyone for whom the video display is obscured or not available). The track is intended to be voiced by a text-to-speech engine.

• chapters — a chapters track includes navigational aid within the resource. If your audio or video is structured in a meaningful way (e.g., scenes), adding a chapters track will enable readers of all abilities to more easily navigate through it.

But now I’m going to trip you up a bit. The downside of the track element that I’ve been trying to hold off on is that it remains unsupported in browser cores as of writing (at least natively), which means EPUB readers also may not support tracks right away. There are some JavaScript libraries that claim to be able to provide support now (polyfills, as they’re colloquially called), but that assumes the reader has a JavaScript-enabled reading system.

Embedding the tracks directly in your video resources is, of course, another option if native support does not materialize right away.