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

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 with a lot of triangle that are too fast to render into an XML tree are also going to be 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.