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Github's flavour is based on Commonmark, with just some extensions on top FYI.

"GFM is a strict superset of CommonMark.", https://github.github.com/gfm/#introduction, third paragraph.

You say that, but look what they are doing with admonitions right now (https://github.com/orgs/github-community/discussions/16925). Despite the RFC (https://talk.commonmark.org/t/generic-directives-plugins-syn...) which is used in a lot of implementations, GitHub decided to do their own thing with an unsemantic, buggy implementation by overloading blockquotes (meanwhile still not supporting admonition styling in reStructuredText or AsciiDoc despite their specs having the feature for years).

That's impossible since there is no invalid markdown/CommonMark in the first place so to add anything you need to change the meaning of something.

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