CLS (Cumulative Layout Shift)

CLS quantifies the annoyance of content jumping around as a page loads — the moment you go to tap a button and an ad pushes it out from under your finger. The usual culprits are images and iframes without width and height attributes, web fonts that reflow text, and content injected above what's already on screen. Reserve space for anything that loads late, and set explicit dimensions on media, to keep shifts near zero.