[ninepoints]

I'd argue that it won the adoption it did in spite of parsing ambiguities and the lack of a spec. Not because of it. There are plenty of examples of well specified things that have gained mass adoption, so I think you are confusing cause and correlation here.

[giraffe_lady]

Well-specified formats that are primarily produced by humans writing them by hand? The main entries in this category are programming languages.

[ninepoints]

IDK, JSON? HTML and XML are markup languages also. There are obvious issues with markdown that were fixed/resolved in various markdown variants, and missing features as well that I don't think anyone could argue helped adoption. Case in point, the most commonly used markdown flavor is GFM because we all adopted GitHub and that was what they support.