The behavior for loading HTML for pages in the MediaWiki namespace should match the behavior of ViewAction/Article.
When trying to load the content of a page in the MediaWiki namespace, and that page doesn't exist, the page/html REST endpoint should look up the system messagwe with the corresponding name and return its value instead. The subopage syntax can be used to reference the message in different languages.
For reference:
- In requests to index.php, HTML display of messages is handled by Article::showMissingArticle
- When editing, the raw text of the message is loaded into the editor by EditPage::getContentObject