For anyone interested, here's the Lemmy markdown configuration. As you can see, Lemmy's website UI supports the full commonmark spec (tutorial / official spec), plus a bunch of extensions. I don't think anyone's fully documented these yet, so I'll try doing so below. Apologies in advance to mobile users, this is probably gonna get ugly (see included image links for how it should look):

• URL autolinking (plaintext URLs automatically turn into links)
• Lemmy autolinking:
  • NOTE: No link will be inserted if the viewer is browsing an instance where the resource is not yet known/blocked
  • Communities: [!fediverse@lemmy.ml](/c/fediverse@lemmy.ml)→!fediverse@lemmy.ml (link ref: /c/fediverse@lemmy.ml)
    • Kbin-style is also supported: /m/fediverse@lemmy.ml→/m/fediverse@lemmy.ml
  • Users: /u/chaorace@lemmy.sdf.org→ /u/chaorace@lemmy.sdf.org (link ref: /u/chaorace@lemmy.sdf.org)
• Typography substitutions:
  • (c)→©
  • (tm)→™
  • (r)→®
  • +-→±
  • ...→…
  • ---→—
  • --→–
  • ???? (>= 4x)→???
  • !!!! (>= 4x)→!!!
• Github-flavor Markdown extensions:
  • Tables
  • Strikethrough: ~~example~~→~~example~~ (image)
• Subscript/Superscript:
  • Sub: example~sub~→example~sub~ (image)
  • Super: example^super^→example^super^ (image)
• Footnotes:
  • Inline part: example[^notename]→example[^notename] (image)
  • Bottom part: [^notename]: Note text→(see bottom of post) (image)
    • Important: the bottom part must be at the very end of the post, otherwise both halves of the footnote will break!
• Ruby Text: {example base text|example ruby text}→{example base text|example ruby text} (image)
  • "Ruby" is an html-ism for special pronunciation aids which frequently appear within young person's media where the language includes non-phonetic characters (e.g.: Chinese characters)
  • Japanese Furigana example: {凄|すご}い！→{凄|すご}い！(image)
• Spoilers:

  visible part example
  hidden part example
  
  

  • See below ⤵️ (collapsed image) (expanded image)
    visible part example

    hidden part example
• Image/Video embedding: ![accessibility alt-text example](https://i.imgur.com/9nVMRqa.jpeg) => (image)

[^notename]: Note text