Defining Cumulative Layout Shift

Cumulative Layout Shift (or CLS) is a measure of how much a webpage unexpectedly shifts during its life. For example, if a website visitor loaded a page and, while they were reading it, banner loads and the page jumps down, that would constitute a large CLS score.

Along with Largest Contentful Paint (the amount of time it takes to load the largest piece of content) and First Input Delay (how long it takes for a page to be interactive or “clickable”), CLS is part of Google’s Core Web Vitals. Google’s web crawlers measure CLS on each page they index.