Understanding the Basics of Cumulative Layout Shift (CLS) in SEO

Cumulative Layout Shift (CLS) is a Core Web Vital metric used in SEO to measure the visual stability of a webpage by quantifying how much visible content unexpectedly shifts during page load or user interaction. It focuses on layout shifts that occur above the fold (the visible part of the page without scrolling) and counts only unexpected shifts—those not triggered by user actions like clicks or taps.

Unexpected layout shifts can frustrate users, for example, when a button moves just as they are about to click it, causing misclicks and a poor user experience. Common causes include late-loading images or ads, dynamically injected content, or fonts loading asynchronously.

CLS is measured as a score representing the sum of all individual layout shift scores during the page lifecycle. A good CLS score is typically below 0.1; higher scores indicate poor visual stability and can negatively impact SEO rankings since Google uses CLS as part of its page experience signals.

To improve CLS, web developers should:

• Reserve space for images, ads, and embeds by specifying size attributes or CSS dimensions to prevent shifts when these elements load.
• Avoid inserting content above existing content dynamically without user interaction.
• Use lazy loading carefully by defining image dimensions to prevent layout jumps.
• Optimize font loading to avoid flash of unstyled text causing layout shifts.

In summary, CLS measures how much a webpage’s visible content moves unexpectedly during loading or interaction, affecting user experience and SEO rankings. Minimizing CLS involves ensuring stable layouts by pre-allocating space and managing dynamic content carefully.