Definition of Cumulative Layout Shift
Cumulative Layout Shift (CLS) measures the unexpected shifting of web page elements while a page is still loading. It quantifies the total movement of content as the page renders, giving developers an idea of how stable the layout is.
Imagine trying to talk to someone who can’t sit still. That’s what a high CLS feels like. Your users are about to click a button when — BAM — it jumps to the left. It’s frustrating, and it’s why CLS is one of the Core Web Vitals. Sites that shift around cause users to miss key calls to action and can even lead to accidental purchases or other errors.
CLS as Part of an Observability Practice
Monitoring CLS as part of your performance observability stack ensures that you catch layout shifts that can sabotage user experience. By catching them early — during development or in staging — you prevent costly mistakes and keep your users focused and engaged.