Reserving ad space dropped CLS from 0.31 to 0.04

Reserving ad space dropped CLS from 0.31 to 0.04

CLS — Cumulative Layout Shift — measures content jumping around while loading. The worst offender is usually an ad or banner that loads late and pushes everything down. Think of it like saving a seat with your coat: if you don't, someone squeezes in and shoves you over.

A media site let ads load into empty space, so the article lurched down 250 pixels when each ad arrived. CLS sat at an ugly 0.31.

The fix reserved the room ahead of time:

— Set a fixed min-height on every ad container matching the ad size.
— The space was held empty until the ad filled it, no shove.

Result: CLS dropped to 0.04.

Try this: scroll your page as it loads. Anything that 'pops in' and pushes text? Give it a reserved height.