CommonMark is the Markdown specification created by John MacFarlane, Jeff Atwood and others, to encompass the various flavours of Markdown that was adopted by different software over the years. GitHub adopted CommonMark along with it's extension for Markdown called GitHub Flavored Markdown (GFM) sometimes around 2016-2017. I've complained about how different platforms deveate from the standard. This GitHub Engineering post shows how good of a job the CommonMark contributors did to represent common usage of Markdown with only 1% deveating from GitHub's previous Markdown parser.

We [GitHub] actually enabled CommonMark for all new user comments in the website several months ago, with barely anybody noticing — this is a testament to the CommonMark team’s fantastic job at formally specifying the Markdown language in a way that is representative of its real world usage.

All in all, less than 1% of the input documents were modified by the normalization process, matching our [GitHub's] expectations and again proving that the CommonMark spec really represents the real-world usage of the language.