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.

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.

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.

Is C amd64 assembly then?

If your C code end up like that, yes:

``` #include <stdio.h>

int main() { /* my software in ASM */ __asm__ ( [...<insert your assembly code lines here>...] );

    return 0 ;

} ```

And you are pretty sure this is pretty much what would happen with markdown in emails if it ended up being mandatory. You would end up with emails entirely made of html.