Presently we say that authors SHOULD NOT use aria-checked on radios and checkboxes.

After discussing w3c/aria#1622 in today's ARIA WG meeting, and the problems that using aria-checked on native checkboxes can have - particularly due to aria-che3cked=mixed and bugs that exist now with Chromium browsers where input type=checkbox checked aria-checked=false will expose 'false' even though the control is actually 'checked', this rule needs to be updated to MUST NOT.