A formal spec for GitHub Flavored Markdown

This post introduces the formal specification for GitHub Flavored Markdown (GFM), based on CommonMark, and details the migration of GitHub's backend Markdown parsing to `cmark`, the CommonMark reference implementation. It covers the implementation of GFM-specific extensions (tables, strikethrough, autolinks, task lists) on top of `cmark`, performance and security contributions to the upstream `cmark` project, and a pragmatic migration strategy involving normalization of existing user content to ensure visual consistency and data integrity. The new system generates an Abstract Syntax Tree (AST) for more robust processing.