Markdown was designed to compile to HTML and web browsers are designed to render HTML. So I’d say a browser is the only reasonable choice.

Emphasis on "Github flavored", which supports additional extensions to the markdown spec that a TUI can't properly preview. You need some kind of GUI (preferably a browser since it does support some html) in order to properly preview it.

I'm assuming it's because the project isn't meant to "Render" markdown, but provide a preview that works offline. That includes hot reloading when the original file changes, which you can't really do with just a client-side library.

If it was a JS library in addition to providing you a webpage with js in it, it would also need to provide you with a server with a hot reload, and we're back to CLI.

They are using goldmark library with some extensions[0] to render markdown. And there's also GitHub API for rendering markdown[1] if you're curious.

References:

[0]: https://github.com/thiagokokada/gh-gfm-preview/blob/main/int...

[1]: https://docs.github.com/en/rest/markdown?apiVersion=2022-11-...