- Backend create/can validation paths emit machine-readable error codes (slug_taken, slug_invalid, content_empty, content_too_large, expiry_invalid, rate_limited, ...) alongside the human message - new.html JS maps codes to plain-language guidance with generic fallback - Result card colored via --ok/--err/--warn left border (result-ok/err/warn) - docs/API.md error section documents the code field - Tests assert the code on every validation path