Actually, this solution doesn’t work, because Parsedown Extra doesn’t support attributes on fenced code blocks.
[Edit:] Ok, somehow the code above doesn’t work, but
```````````````````````````` html
<p>paragraph <b>emphasis</b></p>
````````````````````````````
does. It actually adds a class called .language- plus the custom string to the <code> element. This is probably trying to be a safe implementation of the Markdown extra example.