Why couldn't this be implemented as an extension in an existing browser?

Sending and detecting a different content type (e.g. text/kyu) that triggers (for now) a JS parser that renders the markup used into the HTML that the browser understands

This feels like the best way to get some adoption without having to build a whole new browser

Edit: it also means you could flex away from Markdown and develop your own markup language and parser as and when the need arises