GitHub extensively utilizes Markdown to render wikis, issues, pull requests, and user-generated comments, learning valuable lessons about Markdown parsing through experience. To address vulnerabilities such as DOS attacks, GitHub adapted Natacha Porté's Upskirt library, enhancing it with both feature and security improvements, and developed a Ruby wrapper called Redcarpet as a drop-in replacement for BlueCloth and RDiscount. This library has been successfully deployed across GitHub's frontend machines, and users are encouraged to report any rendering errors for further refinement. In celebration of the new library's release, GitHub Flavored Markdown now supports syntax highlighted code blocks without requiring four-space indentation, allowing users to simply create fenced blocks with backticks and optionally specify a language for syntax highlighting.