We have several core endpoitns that return HTML, such as v1/page/{title}/html. These endpoitns should return HTML for any page, regardless of content model, just like we would for a page view coming in through index.php. Currently, these endpoints only function for content models supported by Parsoid (primarily wikitext).
NOTE: API endpoints should return Parsoid output for wikitext, rather than using the old parser.
Imaplementation note: These endpoints make use of the HtmlOutputRendererHelper class, which in turn makes use of ParsoidOutputAccess. One way to make HtmlOutputRendererHelper work with all kinds of content would be to make it use ParserOutputAccess instead (T317018).