Switching the EditPage buttons to OOUI{1}[2] broke some browser tests[3] because the PageObject elements[3] do not fit the DOM anymore.
[1] https://phabricator.wikimedia.org/T111088
[2] https://gerrit.wikimedia.org/r/#/c/231600/
[3] https://integration.wikimedia.org/ci/job/mwext-mw-selenium-jessie/3018/console
[4] https://phabricator.wikimedia.org/source/mediawiki/browse/master/tests/browser/features/support/pages/edit_page.rb