A change[1] to Special:Notifications to add a "mark as read" button breaks the API with notformat=html, which was deprecated on 2016-02-04[2].
Figure out if it is being used at all or if we can remove it for good.
[1] https://gerrit.wikimedia.org/r/#/c/276256
[2] https://gerrit.wikimedia.org/r/#/c/264718