This is what I don't get: if you have an accessibility layer that is fully usable, that means you have the capability to describe your user interface using only speech/text.

So what are you doing that couldn't be performant in the DOM? Because any kind of fast updates that you're doing that's too fast to render in an XML tree is also going to be too happening too fast to describe with a screen reader?

The DOM is asking you to be able to describe your interface -- performantly -- in entirely pure text. If you can't do that, then there's no way you can be fully accessible, because the screen reader needs to be able to describe your interface -- performantly -- in entirely pure text.

People always get upset about this kind of thing, but it's very obviously true.

If you are updating thousands of rectangles in your UI at 30/60fps, it is impossible for you to fully convey the same amount of information that is changing visually to a screen reader.

Either your interface can be described using pure text, or it can't. Either I could use your interface by closing my eyes and having someone describe it to me, or I can't.