GitHub's spec here is based on [CommonMark][1], which has been around for a while now, and [was originally authored][2] by a group of representatives from GitHub, Reddit, and Stack Exchange.
The first two. [The GFM spec][1] is literally just CommonMark with a few extra extensions added. They even highlighted the new sections green in the spec to make it clear where the GFM spec differs from CommonMark. Everything else is word-for-word identical.
Some quick reading of the linked article says this spec provides a few optional, superset features on top of CommonMark, that it did not contain (like tables, etc).
There was also Common Mark (http://commonmark.org/), which failed IMHO mostly due to John Gruber taking offense at their first choice of name, Common Markdown. Will formalising this as 'GitHub Flavored Markdown' similarly cause offense?
You have to go look at the source code of Grubers implementation to figure out what markdown actually is. Or do some empirical studies with different inputs. That is what I mean by underspecified. His specification is not detailed enough to implement a markdown parser. So in reality it is abondonware.
Did it 'fail'? It seems to have a decent amount of use. And it fixed a bunch of problems with the original Markdown spec (or lack thereof).
I suppose since one of their goals was to get Github and StackExchange 'out of the markdown business' but neither use CommonMark, and further, Github now has put work into creating their own spec, they failed in that aspect.
> one of their goals was to get Github and StackExchange 'out of the markdown business' but neither use CommonMark
FWIW, StackExchange [uses CommonMark][1] for their new StackOverflow Documentation site and has [been planning][2] to migrate Q&A to CommonMark for some time now.
Who knows, but it'd be pretty irrational if it did. Whether it was fair or not, the "Common Markdown" name caused a kerfluffle because of the implication that it was claiming to be the One True Markdown. The name "GitHub Flavored Markdown" only implies that it's base Markdown plus GitHub extras, plus it's been known by that name for a long time now.
