I’ve got something for this by way of adding a d-textarea component. Let me know if this looks good, or if there’s a better ember way to do something like this @joffreyjaffeux 
https://github.com/discourse/discourse/pull/10057
It adds role=“form” to the composer window with the action button’s label (reply/create topic/whisper) so that screenreaders know how to read the element. It also adds the placeholders as aria-labels so screenreaders know how to announce the elements.