MCS should return the same vary header as it got when requesting Parsoid HTML.
For language variants support Parsoid will be the ultimate source of truth regarding whether a certain page can or can not be affected by language variant, so for those pages that can possibly be transformed, Parsoid will set vary: accept-language in its response.
RESTBase will proxy and store that for HTML content, but for MCS RESTBase doesn't want to also fetch in Parsoid HTML in order to determine whether to set the header.
So MCS, when requesting HTML content for transformation should just proxy and return the Vary header it got together with HTML.