role checkbox doesn't support aria-readonly

Hi. ARIA role checkbox [1] doesn't list aria-readonly property, however
it's supported in Firefox. That makes me think aria-readonly on checkbox
was dropped from the spec at some point. I'm curious if anybody recalls
what was the point. Does readonly state make any sense on checkboxes?

Thanks!
Alexander.


[1] http://rawgit.com/w3c/aria/master/aria/aria.html#checkbox