Definition
CLS scores unexpected movement - images without dimensions, late-loading banners pushing content down, fonts swapping mid-read. Users experience shift as mistakes: misclicks, lost scroll position, broken trust.
Why it matters
Layout stability is conversion infrastructure - a button that moves after render steals paid clicks into accidental taps.
Formula
Impact fraction x Distance fraction, summed per session
Benchmarks
Under 0.1 good; over 0.25 poor
All glossary terms