A formal spec for GitHub Flavored Markdown

Starting today, all Markdown user content hosted in our website, including user comments, wikis, and .md files in repositories will be parsed and rendered following a formal specification for GitHub Flavored Markdown. We hope that making this spec available will allow third parties to better integrate and preview GFM in their software.

The full details of the specification are available in our Engineering Blog.

This project is based on CommonMark, a joint effort to specify and unify the different Markdown dialects that are currently available. We’ve updated the original CommonMark spec with formal definitions for the custom Markdown features that are commonly used in GitHub, such as tables, task lists, and autolinking.

Together with the specification, we’re also open-sourcing a reference implementation in C, based on the original cmark parser, but with support for all the features that are actively used in GitHub. This is the same implementation we use in our backend.

How will this affect me and my projects?

A lot of care and research has been put into designing the CommonMark spec to make sure it specifies the syntax and semantics of Markdown in a way that represents the existing real-world usage.

Because of this, we expect that the vast majority of Markdown documents on GitHub will not be affected at all by this change. Some documents sporting the most arcane features of Markdown may render differently. Check out our extensive GitHub Engineering blog post for more details on what has changed.

Written by

Vicent Martí

@vmg