The following properties have boolean or token values, and a value of "undefined" should be treated the same as the attribute not being present:
aria-atomic
aria-autocomplete
aria-busy
aria-checked
aria-current
aria-disabled
aria-dropeffect
aria-expanded
aria-grabbed
aria-haspopup
aria-hidden
aria-invalid
aria-live
aria-modal
aria-multiline
aria-multiselectable
aria-orientation
aria-pressed
aria-readonly
aria-relevant
aria-required
aria-selected
aria-sort
Mozilla has implemented this -- here is the issue in their system:
https://bugzil.la/894573
Comment 1 by aleventhal@chromium.org
, Jul 19 2017