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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I love how the CommonMark Spec has a test suite that's just a JSON array. It's really easy to test a library for compliance, and I've seen developers nerd-sniped into full compliance.
https://spec.commonmark.org/0.31.2/spec.json
It's weird that GitHub shows time since the last commit, but not time since the first commit. It's a nice way of seeing how mature a project is.
One cute feature of markdown I'd not noticed before: there's no syntax for images *without* a description (i.e. alt text).
https://spec.commonmark.org/0.31.2/#images