Cumulative Layout Shift, usually shortened to CLS, is a Core Web Vitals metric for visual stability. It records unexpected movement of visible page content. A high CLS can interrupt reading or cause a visitor to click the wrong control, so it is worth diagnosing as a user-experience issue in its own right.
What causes layout shifts
Common causes include images, videos, embeds or adverts without reserved dimensions; web fonts that change the size of already-visible text; and content added above existing content after the page starts rendering. The important detail is whether a visible element moves unexpectedly, not simply whether a new element exists.
Sources: web.dev: Cumulative Layout Shift
Measure both lab and real-user behaviour
Tools such as PageSpeed Insights and Lighthouse can help identify a layout shift in a controlled run. Where enough real-user data exists, evaluate the 75th percentile separately for mobile and desktop because a lab result alone cannot describe every visitor’s experience.
Sources: web.dev: Optimise Cumulative Layout Shift
Fix the identified shift, then verify it
Reserve layout space for images, embeds and dynamic modules; use font loading and fallback choices that do not reflow the page unexpectedly; and test the actual page after a change. A lower CLS is useful evidence of a steadier page, but it does not by itself prove a ranking or lead improvement.