alt Hacker NewsIt happened a bit differently; Atwood and friends simply came out with a standard document and called it "standard markdown", which Gruber then refused to endorse. Eventually after the series of blog posts and some back and forth they renamed the project "CommonMark", which it is still called today.
I am not sure (of course), but I think Atwood simply thought standardizing this format was so obviously valuable that he didn't consider Gruber might not want to work with him. In retrospect it's kind of nice that it didn't happen, it really keeps everyone incentivized to keep the format simple.
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Here is a post from Atwood about it:
https://blog.codinghorror.com/standard-markdown-is-now-commo...
And an interesting discussion on hn about it: https://news.ycombinator.com/item?id=4700383
The linked post contains three cases of Markdown syntax (underscores) leaking into the text, where actual italics were likely intended. This is the most basic Markdown syntax element failing to work. The problem CommonMark is trying to solve is not adding new features (the only one they added to Gruber Markdown is fenced code blocks), but rather specifying how to interpret edge cases to ensure the same Markdown code produces the same HTML everywhere.