This revision aims to add support for the 'term' and 'definition' ARIA roles.
These roles already exist in Gecko, but aren't fully mapped where they should
be. To address the problem, this revision adds a static atom for "definition,"
implements the ARIA map for definition, adds a markup map entry for the dfn
element (which has the DEFINITION role), and puts the term and definition atoms
in the role map. As a consequence of these changes, this revision also removes
the expected web platform test failures and updates other existing tests.
Differential Revision: https://phabricator.services.mozilla.com/D200219
6b8a3f8047