Render the `dfn` tag

[roke_julian_lockhart (Mr Beedell, Roke Julian Lockhart)]

@merefield

there is a markdown standard for this

@merefield, that CommonMark actually equivalates DL :is(DT, DD).

@merefield

Discourse is not intended to be rendering HTML posts? - it is Markdown based

CommonMark supports all HTML that renders inside the body element, and most that is includable within the head, too. It even hides a prefixed docType.