Extras GitHub adds on top of standard Markdown: tables, task lists, alerts, footnotes, collapsible sections, Mermaid diagrams, mentions and auto-links. Most also work on GitLab and in VS Code.
@username / @org/team: mention a person or team#123: link issue or PR 123; owner/repo#123 for another repo:rocket: style emoji shortcodes- [ ] in issues and PRs become clickable checkboxes and show progressThese extras are not part of CommonMark. If a file is rendered somewhere else (a docs generator, npm, a static site), check that the renderer supports GFM, or stick to core syntax.