HtmlOutputRendererHelper is a helper object for handling rendering of pages that already exist and are stored in the database (ExistingPageRecord) but there is a special case for pages that don't exist but have content (like system message pages in the MediaWiki: namespace) but do not exist in the DB.

For these kind of pages, HtmlOutputRendererHelper does not know how to treat them so we need a class that will handle how to treat such pages.

Maybe call the class I18nHtmlOutputRendererHelper so we know it's for translation messages or SystemMsgHtmlOutputRenderHelper

This patch: https://gerrit.wikimedia.org/r/c/mediawiki/core/+/856549 currently handles how the HTML for this kinds of patches will be rendered but having a cleaner interface for this is nicer.