Steps to reproduce:

1. View data:text/html,<div role="dialog" aria-expanded="true"></div>
2. Examine the states in the Firefox accessibility dev tool or a platform accessibility explorer (e.g. Accerciser)

Expected results: The dialog would lack the expandable and expanded states.

Actual results: The dialog has the expanded state, and also the expandable state on platforms where such a state exists (e.g. ATK).

A change was made in ARIA 1.2 removing aria-expanded as a supported property from a whole bunch of roles. See https://www.w3.org/TR/wai-aria-1.2/#aria-expanded. Dialog is just an example.

Under https://www.w3.org/TR/wai-aria-1.2/#state_property_processing, it says:

User agents MUST ignore non-global states and properties used on an element without a role supporting the state or property;

It would be great if Firefox/Gecko could implement this ARIA 1.2 change.