In 5.4 > progressbar

If the progressbar is describing the loading progress of a particular region of a page, the author SHOULD use aria-describedby to point to the status, and set the aria-busy attribute to true on the region until it is finished loading.

This statement leaves it somewhat unclear whether the author should put the aria-describedby attribute on the region of the page that is loading and point it to the progressbar or vice-versa. Contrast this with the instructions for aria-busy, where it is made clear that attribute should be placed on the region and guidance is given on what to do regarding the attribute when the loading has finished.

TLDR: clarify which element aria-describedby should go on and which element it should point to in this case, and what should be done when the loading finishes.

Related: mdn/content#10707