By the way, while browsers can be both CPU and memory hogs, they're not inherently inneficient and it mostly depends on what the website is doing. This project uses minimal JavaScript and no frameworks, and while I didn't measure it I assume this site should be reasonably efficient, both in memory and CPU usage.
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-...
(nit: hot reload means a specific thing in the js ecosystem and you prob dont mean that)