CommonMark

Although the term Markdown is probably more familiar to most users, Markbook actually uses CommonMark.

CommonMark is a formally defined standard, unlike Markdown, which was originally nothing more than a perl script for a format that contained numerous parsing errors that newer implementations were forced to replicate.

CommonMark represented an industry-wide attempt to define a common markdown format, and included efforts by the creators of StackEdit and Pandoc. GitHub rewrote their GitHub Flavored Markdown (GFM) format to be an extension of CommonMark.