There is an ongoing effort to standardise Markdown; I think it would be nice to check whether this implementation is compliant with it.

IkiWiki's mdwn plugin does not contain an implementation of Markdown: it relies on external libraries. It can currently use any of these, most-preferred first:

In practice, Discount is the implementation pulled in by the Debian package dependencies, and (I suspect) the most commonly used with IkiWiki.

If the selected external library (whatever it happens to be) complies with a particular interpretation of Markdown, then IkiWiki will too. If not, it won't. The only influence IkiWiki has over its level of compliance with a particular interpretation is in how we choose which external library we prefer.

As such, if you want IkiWiki to change its interpretation of Markdown, the way to do that is to either change Discount's interpretation of Markdown, or contribute a patch to make prefer a different (and presumably "more compliant") Markdown implementation.

IkiWiki has one syntax extension beyond Markdown, which is that text enclosed in double-square-brackets is an IkiWiki wikilink or directive. This applies to any markup language used with IkiWiki, not just Markdown.

(There also doesn't seem to be any consensus that labelling any particular fork of Markdown as "standard" can make it the truth, or that this particular fork is the Correctâ„¢ fork and not just; but that's between the authors of Markdown implementations and those who want to standardize Markdown, and it isn't IkiWiki's job to police that.)