@aslakrAslak Raanes As far as I know, there's no such thing. 🤔
@hongminhee洪 民憙 (Hong Minhee) 
Maybe activitypub implementations like Mastodon should not filter out https://html.spec.whatwg.org/multipage/microdata.html like `itemscope`, `itemtype` etc.?
One could then use something like https://schema.org/SoftwareSourceCode indicating that there is a source code block that could be highlighted for the user.
If you have a fediverse account, you can quote this note from your own instance. Search https://mastodon.social/users/aslakr/statuses/114627520850496574 on your instance and quote it. (Note that quoting is not supported in Mastodon.)