We have to update our data-mw spec for expanded attributes since multiple attributes from a template seems quite common.

Ex: https://hi.wikipedia.org/wiki/%E0%A4%B8%E0%A4%BE%E0%A4%81%E0%A4%9A%E0%A4%BE:Yearheader

See commit message of https://gerrit.wikimedia.org/r/#/c/173834/ for more information and a failing wt2wt test because of the gap in our DOM spec. This requires co-ordinating with VE to make sure they can handle the updated spec. More broadly, will require alerting DOM clients on wikitext-l to make sure they can update their code.