This revision implements the mapping for the ARIA 1.2 emphasis role, which was
unsupported in Firefox until now. This change addresses a web platform test
failure. To accomplish this, the revision adds the WAI-defined role, adds a role
enum value, adds platform mappings, and adds a markup role mapping. The change
requires a new static atom for the word "emphasis," also added in this revision.
Finally, this change removes the expected wpt failure and updates other tests.
Differential Revision: https://phabricator.services.mozilla.com/D200130
