The spec at 0.25 apparently has no references section. It SHOULD list RFC 7763 + 7764. The former registered MIME type text/markdown with suggested file extensions *.md or *.markdown, that’s a good thing.
The latter discusses [CommonMark](https://tools.ietf.org/html/rfc7764#section-3.5) with [examples](https://tools.ietf.org/html/rfc7764#section-4.5) among other flavours (Github GFM, pandoc, etc.)
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(Circumventing new user spam protection with a hammer, and why on earth do three tildes not work here, backticks are horrible _dead keys_ on European keyboards.)
Updated JFTR, RFC 7764 already [registered](https://www.iana.org/assignments/markdown-variants/markdown-variants.xhtml) **CommonMark** as text/markdown **variant**, a direct link to the IANA registry _SHOULD_ be also added to the spec.
BTW, I'm unable to add keywords IANA + MIME in Chrome, the "save edit" button is hidden behind a not helpful list of existing keywords consisting solely of "release 1.0".