The "accesskey" element is part of XForms, and can be
added to any "form controls"
(http://www.w3.org/TR/xforms/slice13.html#def-form-control),
according to
http://www.w3.org/TR/xforms/slice8.html#id2625797.
Adding one to a trigger results in an error (code 1) in
the validator.
Example attached.
Example XForms