As described in #1585 (comment) as well as in ARIA WG meetings 1 and 2, what implementors have done for ARIA reflection does not appear to match what is in the spec.

In particular, default/implicit values for missing attributes are not being reflected, and no validation is taking place. When an attribute is not present in the DOM, the reflected value is null. Thus as it currently stands, I believe we have 0 implementations for the IDL interface as spec'ed in ARIA 1.2.

What is currently implemented seems consistent with the properties being a nullable DOMString. Therefore, unless there is a strong belief that what is in the spec now (attributes are a non-nullable DOMString) is correct and that the implementations are wrong, the thing to do might be to update ARIA 1.2 to reflect (pun intended) reality.