[arp242]

It's not. Markdown is markdown. It can render to HTML, but that's a different thing. I struggle to understand why this is even a point of contention.

[prmoustache]

Since it supports any HTML tag, it is irrelevant.

Mandate markdown and MTAs and marketing departments will send you markdown only made of pure HTML.

[arp242]

That's quite the pivot from "markdown is HTML". But no, it probably shouldn't support HTML tags, or support it with limits. And maybe also a few other things like #-headers also shouldn't be supported as it can be too easily to do by accident. All of that seems pretty obvious in all but the most aggressively pedantic reading.