Try to fork the repo and run it locally. There's a button for creating cards (button appears only when running it locally). It's still HTML-in-JSON, but you get instant preview while editing, and I think it's a nice way to format cards. HTML offers more flexibility in formatting than Markdown does. That said, I totally understand you might prefer a different format, and if you want to work on it, I'm sure you will be able to replace the HTML-in-JSON format with Markdown.