GitHub Flavoured Markdown now has a formal spec as a superset of CommonMark! https://githubengineering.com/a-formal-spec-for-github-markdown/
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Terminad is a really interesting approach to rendering markdown in the CLI. It deliberately doesn't render links, so you can always see both the link text and the URL.
In a browser you have hover to see URLs, but not necessarily in a terminal.
One cute feature of markdown I'd not noticed before: there's no syntax for images *without* a description (i.e. alt text).

Today I learnt that you can mix HTML inline in markdown! For example, the following is valid.
Foo <hr/>
I'd assumed that you needed HTML separately, like ``` blocks, but no: https://spec.commonmark.org/0.31.2/#raw-html
Admittedly HTML is very restricted on most sites, but it's helpful for SSGs.