Board Thread:Questions and Answers/@comment-8072-20151230003114/@comment-10473115-20160327193206

In Template:Infobox/Promotion navigation tags belong to portable infobox syntax (Help:Infoboxes/Tags) and I guess that loading w:c:developers:PlusOneButton enables div class="g-ytsubscribe" to work site wide. See also.

It seems that verbatim tags work for us but we can't edit or create corresponding MediaWiki pages so only those existing uses are grandfathered in. Twitter button, however, is created with javascript loaded in MediaWiki:Common.js. This page could help you to implement it.