Accessibility of model

We need to figure out how to make <model> accessible on a number of different fronts:

• Visual: describe what is being presented over time.
• Interaction: describe what can interacted with (regions, buttons, etc.).
• auditory: describe audio and possibly spoke sounds, potentially over time.

Usually, this would be provide by the embedded format... however, it appears that both glTF and USDZ are quite limited when it comes to accessibility.

As such, it may be that we need to leverage what we can from HTML + ARIA to overcome the shortcomings of these formats. We have quite a bit of precedent (e.g., from the humble, yet limited, alt attribute, to how <canvas> can be made accessibly, to the potential inclusion of <track> elements, and so on).

The issue names no files or tests; begin by reviewing the accessibility requirements alongside the mentioned glTF, USDZ, HTML/ARIA, canvas, alt, and track precedents. Because it proposes several possible directions without acceptance criteria, establish the concrete scope and definition of done before implementation.