Definition
CLS (Cumulative Layout Shift) is a Core Web Vital metric that measures visual stability by quantifying the amount of unexpected layout shifts occurring during the loading and interaction phases of a webpage. It evaluates how much visible content shifts without user input, with scores ranging from 0 to 1, where lower scores indicate better stability. CLS has become a significant ranking factor as part of Google’s Page Experience signals, directly affecting how websites perform in search results.
Cumulative Layout Shift identifies instances where page elements move unexpectedly as a page loads, causing frustration when users attempt to interact with content that suddenly shifts position. These shifts commonly occur when resources load asynchronously or DOM elements are dynamically added above existing content, creating a poor user experience when users attempt to click links or buttons that move before the interaction completes.
Key characteristics of CLS include:
- Measurement based on the sum of individual layout shift scores throughout the lifespan of a page
- Calculation involving both the visible impact fraction (portion of screen affected) and distance fraction (distance elements move)
- Classification into “good” (less than 0.1), “needs improvement” (between 0.1 and 0.25), and “poor” (above 0.25) categories
- Direct impact on search rankings as part of Core Web Vitals
- Assessment in both lab environments (controlled testing) and field data (real-user monitoring)
- Particular importance for mobile experiences where screen constraints amplify the impact of shifts
- Exclusion of shifts that occur within 500ms of user interaction (as these are considered expected)
- Focus on unexpected movement rather than intentional transitions or animations
- Representation as a unitless value reflecting the worst movements in the user experience
History of CLS
The concept of measuring layout stability has evolved alongside Google’s increasing focus on user experience:
2019 (May): Google first introduces the concept of layout shift metrics at the Google I/O conference, signaling a shift toward quantifying previously subjective aspects of user experience.
2020 (May): Google officially announces Cumulative Layout Shift as one of the three Core Web Vitals metrics, alongside Largest Contentful Paint (LCP) and First Input Delay (FID).
2020 (November): Google confirms that Core Web Vitals, including CLS, will become ranking signals in 2021 as part of the Page Experience update, giving webmasters time to prepare.
2021 (April): The CLS metric is refined to use a windowing approach that prevents single-page applications and long-lived pages from accumulating unreasonably high scores over time.
2021 (June): The Page Experience update begins rolling out, officially making CLS a ranking factor in Google’s search algorithm.
2022-2023: Further refinements to CLS measurement methodologies occur, with better handling of intentional layout shifts and improved guidance for developers.
2024-2025: CLS remains a critical component of user experience evaluation as Core Web Vitals continue to evolve with the addition of new metrics while maintaining focus on visual stability.
Types of CLS Issues
CLS problems manifest in various patterns across different website types:
Image Dimension Issues: Images without specified width and height attributes causing content to shift once the visual loads.
Ad Container Shifts: Advertisements loading dynamically and resizing their containers, pushing surrounding content.
Font Display Problems: Web fonts rendering differently than their fallbacks, causing text reflow when the custom font finishes loading.
Injected Content Shifts: Third-party widgets, social media embeds, or cookie consent notices inserting content and pushing existing elements.
Dynamic Content Loading: AJAX-loaded content appearing after initial render, pushing down previously loaded elements.
Banner/Notification Shifts: Promotional banners or notification systems appearing after page load, displacing main content.
Responsive Element Reflow: Elements that change size or position at different viewport widths when resizing a browser window.
Animation-Induced Shifts: Improperly implemented animations that affect the position of other page elements.
Lazy-Loaded Component Shifts: Improperly implemented lazy loading causing surrounding content to reposition when off-screen elements load.
Form Validation Shifts: Error messages or validation notices appearing and shifting form fields after submission attempts.
Importance in Modern SEO
CLS has become increasingly critical in contemporary SEO practice for several compelling reasons:
As a direct ranking factor within Google’s Page Experience signals, CLS directly influences how websites perform in search results. Google has explicitly confirmed that sites with poor Core Web Vitals scores, including CLS, may see negative impacts on their rankings, while sites with good scores gain a competitive advantage, particularly in cases where content relevance is similar between competing pages.
The user experience implications of CLS extend beyond SEO to affect key business metrics including conversion rates, bounce rates, and customer satisfaction. Research shows that visually unstable pages frustrate users, with Google finding that 24% of visitors abandon sites with significant layout shifts. This direct connection between CLS and business outcomes makes it relevant beyond just search visibility.
Mobile optimization makes CLS particularly crucial, as limited screen space on mobile devices amplifies the negative impact of shifting elements. With mobile-first indexing now standard and mobile traffic dominating many sectors, addressing CLS issues represents a significant opportunity to improve both rankings and user experience for the majority of visitors.
The technical focus on CLS has pushed web development practices toward more stable rendering approaches, including reserving space for dynamic elements, properly sizing media assets before load, and implementing content in ways that don’t cause disruption. This has elevated technical SEO’s role in development processes and strengthened the connection between user experience design and search optimization.
For large enterprise websites or content management systems, systematic CLS issues often affect thousands of pages, creating scaled opportunities for improvement. Identifying and addressing these patterns can yield significant overall performance gains across large sites, making CLS optimization a high-leverage activity for SEO teams managing substantial digital properties.
As Google continues refining its algorithms to prioritize user experience, metrics like CLS may gain even greater importance in the competitive landscape of search rankings. Early adopters who build technical systems and workflows that prioritize visual stability position themselves advantageously for future algorithm updates that may place increased emphasis on these user-centric metrics.
For SEO practitioners, the inclusion of CLS in Core Web Vitals represents a shift toward more holistic optimization that bridges technical performance, user experience design, and traditional search visibility tactics. This evolution expands the scope of SEO beyond keywords and links to include the quality of interaction with content after discovery.Retry