Markdown? Terrible "spec".

Browsers already support XML.

You can spin up a HTML-but-restricted XML grammar (with extra stuff even, like footnotes and stuff) and a CSS file in maybe half an hour, and it'll render in your browser just fine.

(Yeah, it'll be missing all the accessibility provisions, but you know, the base to build on is there, whereas "MarkDown in the browser" rendering has been often suggested and never implemented).