Cumulative Layout Shift

Cumulative Layout Shift is a numeric score for visual stability. It measures every unexpected layout shift that happens after the page begins to render — an image popping in without reserved space, a banner ad pushing the article down, a button appearing in a different position than expected. The score multiplies the size of the shift by the distance the elements moved.

Good CLS is under 0.1. It’s usually the easiest Core Web Vital to fix because the causes are mechanical: missing `width` and `height` on images, web fonts that swap to a different metric, dynamically inserted content (chat widgets, cookie banners) that doesn’t reserve its space.

Beyond the score, CLS is worth caring about because it’s the metric users feel without realizing it. A site that doesn’t shift on load *feels* well-built.