Cumulative Layout Shift (CLS) is the Core Web Vital that kills user trust silently. Pages that jump around as

Cumulative Layout Shift (CLS) is the Core Web Vital that kills user trust silently. Pages that jump around as they load see 15-25% higher bounce rates even when overall load time is acceptable.

The main culprits:

— Images without explicit width/height attributes. The browser doesn't know how much space to reserve.
— Ads injected above content after load. Late-loading ad units are the #1 CLS killer on affiliate sites.
— Web fonts causing text reflow. FOUT (Flash of Unstyled Text) causes layout shifts.
— Dynamically injected banners or cookie consent popups that push content down.

The fix is boring but effective: set explicit dimensions on every media element, reserve space for ads with min-height containers, and use `font-display: optional` for non-critical fonts.

Target CLS below 0.1. Anything above 0.25 is penalized in Google's ranking signals. Run a field data test via CrUX, not just lab data — real user sessions often show higher CLS than lab tools capture.