<b>Reserving ad slot height to kill CLS</b>

<b>Reserving ad slot height to kill CLS</b>

What it does: sets a fixed min-height (or aspect-ratio box) on every ad container so the page doesn't jump when the creative loads.

Setup effort: 1/5 — CSS only. The catch is multi-size slots: you must reserve for the tallest expected creative.

RPM lift seen: indirect. CLS over 0.1 hurts rankings and pogo-sticking; fixing it protects 5-15% of organic sessions you'd otherwise leak. No direct CPM change.

Pros
— Cheapest Core Web Vitals win there is
— Stops the rage-misclick that poisons CTR data

Cons
— Over-reserve and you get ugly empty gaps on unfilled slots (collapse-on-empty fixes this but reintroduces shift)
— Fluid/responsive sizes make 'tallest' a guess

Verdict: reserve for the dominant creative size, collapse only truly empty slots after a timeout. Lift: ★★★☆☆ — Risk: ★☆☆☆☆
Best for: every site. There's no one who should skip this.