Three GFM extensions that show up in every PR
Strikethrough
Wrap text in double tildes: ~~deprecated~~. Single tildes do nothing in GFM. Strikethrough is the polite way to mark removed steps in a checklist or deprecated values in a doc — the original is still readable, but the visual cue is unmistakable.
Autolinks for raw URLs
In CommonMark, you have to wrap raw URLs in angle brackets — <https://example.com> — to get a link. GFM auto-detects bare http://, https://, ftp://, and www. URLs in normal prose and turns them into links. No syntax required.
GitHub-only references
On github.com, GFM extends autolinking further: @username mentions a user, @org/team mentions a team, #123 links to issue/PR 123, and a 7+ character SHA links to the commit. These are not portable — they only render outside GitHub if you write the long form yourself.
Email autolink trap: GFM also auto-links bare email addresses (
name@example.com). If your README contains a literal email and you don't want it linked, escape with backticks (`name@example.com`) or wrap in HTML (<span>name@example.com</span>).