"The lack of a standard helps keep people honest by making them follow the spirit of the law, not the letter, as you see happening with things like HTML and CSS."

I recognise that line of reasoning from the RSS 2.0 days. And how RSS feeds were okay for years until Dave Winer decided one day that they were funky. Having a deterministic standard is a quality metric.

On Gruber's podcast: he notes that writers should check whether something is valid Markdown by rendering it and seeing the output ( https://overcast.fm/podcasts/episode/344902019595#t=4527 ). paraphrasing his logic: If the output is wrong, that means the input needs fixing. That is an assertion that the code implementation is the specification. And we've been down that road before ( bugwards compatible implementations of Internet Explorer, for example )

Ambiguity is not a good basis for a data structure specification. An undefined behaviour means that all possible behaviours are correct.