CLS (Cumulative Layout Shift)
CLS measures unexpected layout movements during page load: the button that jumps under a finger, the text that shifts when an image loads. Below 0.1, the experience is considered stable.
In practice
Causes are few: images without declared dimensions, font swaps, injected banners, ads without reserved space. Each has a clear fix — explicit dimensions, controlled font loading, reserved slots. An absolute zero is achievable; our templates demonstrate it in production.