The Content class has a getWikitextForTransclusion() method, possibly CssContent/JavaScriptContent/JsonContent should override it with… something.

Well, @matmarex, what do you think about (in pseudocode)

return $('<syntaxhighlight>', {lang: 'css', html: super()});

will it work?

Yes, basically, with two caveats:

• SyntaxHighlight is an extension, so MediaWiki core would have to use <pre> and add a hook that SyntaxHighlight can use to change it to <syntaxhighlight>.
• MediaWiki's XML-like tags are not nestable, so if the page being transcluded contained </pre> itself, the styling would stop in the middle of the page. This can probably be avoided with some clever parser abuse.

You are the boss, @matmarex. I also added the lua model.

Parsoid would probably also need something done here, if only replacing whatever it currently does to get the wikitext for a transclusion with something that calls getWikitextForTransclusion(). And in turn that might need some change to the action API.

In T146771#3091591, @Anomie wrote:

Parsoid would probably also need something done here, if only replacing whatever it currently does to get the wikitext for a transclusion with something that calls getWikitextForTransclusion(). And in turn that might need some change to the action API.

Parsoid calls the expandtemplates action api endpoint, so as long as that does the right thing, this should work in Parsoid.