https://www.markdownguide.org/extended-syntax/

This has most of the extended syntax, including footnotes[^1]. I use the syntax highlighting quite often, and a few markdown editors/previewers support mermaid.js which is pretty cool if you need a simple diagram and you don't want to mess around with a more complex app.

[^1]: If you are talking about Lemmy formatting specifically, I don't think it's feature complete yet, but it does support some things that aren't documented

I always thought that habitat67 was a good example of how to do add density in a way that didn't feel dense. It's too bad this never got further than Expo 67.