You're basically advocating for text/richtext (aka text/enriched), which was originally proposed explicitly as a replacement for HTML in email. It failed essentially because everyone who didn't want HTML wanted only plain text, and everyone who wanted something like HTML was happy to just use HTML.

Factoid of the day: Netscape added support for a <blink> tag to text/enriched bodies, and that support remains in every client that's still using Netscape's MIME code. The comment above this code just says: "Of course, both text/richtext and text/enriched must be enhanced somehow... Or else what would people think."

No markdown, but we had success with this one: https://mjml.io/

Try out the online examples.

Something like this? http://www.rfc-editor.org/rfc/rfc1896.txt

Markdown is nice for composing, but not as a standard to be supported by many clients. CommonMark is an improvement, but there is way to many ambiguities and differences in supported features over different markdown parsers for it to be feasibly used as a standard.

Markdown is also not any better than HTML in terms of security - most markdown parsers allow passing through arbitrary HTML, and then depend on sanitizing the HTML for security.

What you are asking for is effectively a smaller subset of HTML to be used for for email (which is kind of how it started...)

Try Foundation https://foundation.zurb.com/emails.html

I've used it on several projects, it speeds things up quite a bit

Beside, I am using Markdown in all my email thanks to https://markdown-here.com/ and Thunderbird

The idea of markdown is great because it it essentially a text. Even if the client would not support it, humans would be able to read it just fine. Much better and more secure than rich HTML. For example my email client won't display images by default and I am happy with that. Rich HTML should be on the web only IMO!

This + multipart/alternative email and maybe something like uMatrix or similar firewall mechanisms in mail clients

But isn't Markdown a sugar on top of HTML? When something does not fit Markdown one is supposed to write inline HTML.