I'm never seen the `::: header` or `{width="200"}` kind of syntax before. Is this custom or Frankenstein solution? Or is there some kind of md-extended pattern for defining components that has been gaining steam or smthn? Markdown tooling is always confusing, since everyone has their own standard.
Admonitions are :::, allows you do to do things like this, if you have custom parser, if not, your admonitions can fail gracefully as plain text
:: gallery
  
::
https://github.com/remarkjs/remark-directive
https://talk.commonmark.org/t/generic-directives-plugins-syn...
Yep both are widely used. I forget which markdowns extensions they originated from. The Pandoc website probably has the details.
Aye, I've seen these both in kramdown and quarto.