Measures your monitor's actual refresh rate via browser rendering pipeline.
Measuring... ensure browser is focused and not limited by power saving.
Track the moving block with your eyes. It should move fluidly without judder or skipping.
Disclaimer: This is a browser-based estimation using requestAnimationFrame. It is limited by your browser's VSync lock. For example, Safari may lock to 60fps on a 120Hz display if low power mode is on.