Kill CLS layout shifts in order of impact

Kill CLS layout shifts in order of impact
CLS (Cumulative Layout Shift = sum of unexpected movement) is driven by a few big shifts, not many small ones.

— Step 1: Performance panel > Experience track shows each shift + its score.
— Step 2: Sort: one shift of 0.18 beats twenty of 0.001. Fix the giant first.
— Step 3: Images/iframes/ads: set explicit width+height or aspect-ratio. No reserved box = the top cause.
— Step 4: Fonts: use font-display:optional or size-adjust to stop the swap reflow.
— Step 5: Inserted banners: reserve min-height; never push content down.

Example: one unsized ad slot scored 0.21 of a 0.24 total.

Takeaway: CLS passes at p75 under 0.1 — eliminate the single largest shift first.