Support `amp-story-captions` for `amp-audio`

Description

In our Web Stories editor we want to leverage amp-video to play audio files, so that we can display captions for them.

To achieve that, we basically use amp-video like this (simplified):

<amp-video layout="nodisplay" autoplay captions-id="caption-el">
  <source src="https://example.com/audio.mp3" type="audio/mpeg" />
  <track src="https://example.com/track.vtt" />
</amp-video>

This works great and the audio plays fine and the captions are displayed correctly.

However, we do get a validator warning that the video requires a poster image:

The mandatory attribute 'poster' is missing in tag 'amp-video

I think in such a case — the video being hidden and only playing audio — the poster requirement could be lifted

Alternatives Considered

Two alternatives I could think of:

1. Add amp-story-captions support to amp-audio and let AMP render a <video> for the audio element in the background
2. Just provide the URL to some dummy poster image (accessibility concerns?)

Additional Context

No response

[gmajoulet]

We’re actually not 100% sure about video caching for audio files, but have been looking into it as of next week. If it doesn’t work we’ll fix it very soon.

[newmuis]

1. The real answer is to use <amp-audio> and <amp-story-captions>, as you imply. I’m a bit surprised that <amp-story-captions> does not support <amp-audio> already; @gmajoulet do you have a sense of what more is required to support it?
2. Given that you need to use <amp-video>, I think it may actually still be valid to require a poster in this case. The <amp-video> tag will be visible to end users (so that the captions show up), and until the audio source and/or captions have loaded, end users will be looking at a blank HTML element. If the component is meant to introduce visuals, we need some kind of placeholder content to show until that visual is ready to be displayed. If the creator (or a tool, on behalf of the creator) can be certain that the visual is not needed, then I think your second workaround is probably best in the interim.