Definition
Cumulative Layout Shift (CLS) measures unexpected layout changes that occur during a webpage's loading process, tracking how much visible content shifts without user interaction.
Use Cases & Examples
Core Web Vitals Visual Stability Measurement
CLS is one of Google’s three Core Web Vitals metrics that directly impacts search rankings and user experience. Google considers CLS scores under 0.1 as “Good,” between 0.1-0.25 as “Needs Improvement,” and over 0.25 as “Poor.” Unlike other performance metrics that measure speed, CLS focuses on visual stability by calculating how much content moves unexpectedly as the page loads, providing a quantitative measure of layout disruption.
Common Layout Shift Causes
Layout shifts typically occur when content loads without reserved space, causing existing elements to move. Common culprits include images loading without specified dimensions, web fonts changing from fallback fonts, ads or embeds inserting dynamically, content injection from JavaScript, and CSS that affects element positioning after initial render. Each shift is scored based on how much of the viewport is affected and how far elements move.
User Experience and Interaction Disruption
Poor CLS creates frustrating user experiences where content jumps around unexpectedly, often causing users to click the wrong buttons or lose their reading position. For example, a user might try to click a “Subscribe” button, but an advertisement loads above it, shifting the button down and causing the user to accidentally click an ad instead. Good CLS ensures visual stability, allowing users to interact confidently with page content.
Mobile Impact and Touch Interactions
CLS issues are particularly problematic on mobile devices where users interact through touch. Layout shifts can cause accidental taps on wrong elements, misplaced form submissions, or disrupted scrolling experiences. Mobile users are more sensitive to layout changes due to smaller screens and finger-based navigation, making CLS optimization crucial for mobile user experience and engagement.
Common Misconceptions
“CLS only measures image-related layout shifts”
CLS measures all unexpected layout changes, including those caused by fonts, ads, injected content, CSS changes, and any element that moves without user interaction.
“A CLS score of 0 is always achievable”
While a score of 0 is ideal, some layout shifts may be unavoidable depending on content and functionality. The goal is to minimize shifts and stay within Google’s “Good” threshold.