React Guideline: Forms — Controlled components for forms. Description: Use state to control form inputs. Do: value + onChange for inputs. Don't: Uncontrolled inputs with refs. Good Example: <input value={val} onChange={setVal}>. Bad Example: <input ref={inputRef}>. Severity: Medium. Docs: https://react.dev/reference/react-dom/components/input#controlling-an-input-with-a-state-variable.