Problem
Messages currently have a role="contentinfo" wrapper for status messages (compared with role="alert" for error messages).
contentinfo is inappropriate here, because it's supposed to be for information about the document as a whole, not status updates. It's a landmark role, and there should only be one such contentinfo region per page. It's intended to serve the same purpose as a top-level HTML footer element. Using contentinfo for messages may be present some confusion for users who navigate by landmark regions.
Proposed resolution
Use role="status" as the wrapper for messages instead. Like role="alert", it is an ARIA live region, so assisitive technology can treat them accordingly. Some (not all) browser + screen reader combinations announce these regions after a page loads, so users will have a quicker understanding of what happened after submitting a form.