Cumulative Layout Shift (Cls) Fixes: Complete Guide 2025

Answer: Cumulative Layout Shift (CLS) fixes stabilize visible content during page load by reserving space, defining image dimensions, optimizing fonts, and controlling third‑party scripts to prevent unexpected shifts, improving user experience and Core Web Vitals performance across devices, viewports, and platforms.

Cumulative Layout Shift (CLS) fixes reduce unexpected visual movement that disrupts users during page load and interaction. This guide presents a systematic approach: define CLS, explain measurement, identify root causes, and deliver practical, code-level fixes for images, embeds, fonts, CSS/JS, and third-party scripts. The introduction outlines why CLS matters for search ranking, conversion, and mobile retention, and previews a prioritized remediation plan you can apply in audits, sprints, and CI pipelines. Core Web Vitals set a performance threshold where a CLS score of 0.1 or lower is considered good, 0.1–0.25 needs improvement, and above 0.25 is poor. This guide uses field and lab measurement principles, demonstrates platform-specific implementations for WordPress, Shopify, and Next.js, and includes two detailed case studies with measurable outcomes. Practical examples include HTML/CSS patterns for reserving space, ad-slot stabilization strategies, font preload and font-display workflows, and third‑party script containment. Performance testing actions include Lighthouse audits, Chrome User Experience Report interpretation, and Web Vitals JavaScript for Real User Monitoring (RUM). Implement the quick-start checklist within 24–72 hours to reduce the largest shifts, then integrate monitoring, budgets, and regression checks into CI. This article reflects recommendations from Google Developers and web.dev guidance and consolidates implementation patterns used on mid-size and enterprise e-commerce sites. Expect initial CLS reductions by targeting the top three shift causes first: images and media, ad and embed slots, and late-inserted UI elements. The remainder of the guide lays out step-by-step remediation, testing workflows, maintenance practices, and future trends in automation and browser features that will affect Cumulative Layout Shift (CLS) fixes.

Cumulative Layout Shift (CLS) fixes: Definition & Overview

Cumulative Layout Shift (CLS) fixes address the metric that measures unexpected layout movement of visible page elements during page lifecycle. CLS quantifies visual stability by aggregating layout shift scores across page loads and interactions to represent how much content moves for users.

CLS originated as one of Google’s Core Web Vitals to capture visual instability that harms user experience and trust. The metric isolates layout shifts that are not caused by user input and aggregates the impact into a session-level score. A stable page preserves spatial context; layout shifts erode readability, cause mis-clicks, disrupt forms, and reduce conversion rates.

Key attributes of CLS:

• Score composition: per-shift Impact Fraction × Distance Fraction contributes to the total CLS score.
• Session aggregation: multiple shifts across the page lifecycle sum to the final score used in field reports.
• Excludes user-initiated shifts: shifts caused by user interactions are ignored if they are directly attributable to interaction events.
• Viewport sensitivity: impact depends on visible area affected relative to viewport size.

Why implement Cumulative Layout Shift (CLS) fixes now:

• Search engines include CLS in ranking factors via Core Web Vitals, so improvements influence organic visibility.
• Reduced shifts lower user friction on mobile where screen size amplifies perceived instability.
• Stabilized layouts reduce accidental clicks and form errors, supporting conversion rate optimization.

Key takeaway: CLS is a measurable expression of visual instability; targeted Cumulative Layout Shift (CLS) fixes remove sources of unexpected movement to protect UX and performance metrics.

How CLS Works: Calculation, Sampling, and Thresholds

CLS is calculated by summing layout shift scores, where each shift score equals Impact Fraction multiplied by Distance Fraction. The Impact Fraction measures the area of the viewport that was affected by the shift; the Distance Fraction measures how far elements moved relative to the viewport.

Calculation mechanics:

1. Identify a layout shift: any movement of visible elements not caused by user input during page lifecycle.
2. Compute Impact Fraction: fraction of viewport area impacted before and after the shift.
3. Compute Distance Fraction: the greatest relative movement distance of unstable elements divided by viewport dimension.
4. Multiply Impact Fraction × Distance Fraction for per-shift score; aggregate all shift scores to determine session CLS.

Sampling and aggregation:

• Field CLS uses Real User Monitoring (RUM) via Chrome User Experience Report (CrUX) and Web Vitals instrumentation to capture real sessions.
• Lab tools like Lighthouse simulate page loads and report a synthetic CLS score that approximates expected behavior under test conditions.
• CLS in CrUX represents aggregated user sessions over a 28-day window for origin-level reporting.

Thresholds and classification:

• Good: CLS ≤ 0.10
• Needs improvement: CLS > 0.10 and ≤ 0.25
• Poor: CLS > 0.25

Practical measurement notes:

• Lab CLS can under-report shifts triggered by late-loading third-party content; include RUM to capture those effects.
• Single large shifts have more impact than many small shifts; prioritize fixes that eliminate high-impact shifts first.

Key takeaway: CLS uses precise geometric calculations and session aggregation; combine lab and field data to identify and prioritize Cumulative Layout Shift (CLS) fixes.

Causes & Signals: What Produces Layout Shifts

Common causes of layout shifts include images without dimensions, dynamically injected ads or embeds, slow font loading, late DOM insertions, and asynchronous third-party scripts. Each cause produces characteristic signals that allow targeted remediation.

Images and media

Images and video that lack explicit width/height or CSS aspect-ratio cause the browser to allocate no or incorrect layout space, producing a shift when the media loads. Responsive images without aspect containers produce visible reflows on load or on viewport changes.

Ads and embeds

Ads and third-party embeds frequently insert variable-sized content after initial rendering. Ad creatives that change height or load asynchronously without reserved slots generate large impact fractions.

Font loading

Custom fonts can cause FOUT/FOIT behavior that changes text metrics; a change in font metrics after initial paint moves inline content and block layout, producing layout shifts across multiple elements.

Dynamic content and DOM mutations

JavaScript-driven content insertion, such as client-side rendering frameworks mounting components, rendering skeleton loaders incorrectly, or injecting banners, alters layout flow and can create shifts.

Transitions and animations

Animations that change layout properties (width, height, margin) trigger shifts when not implemented using transform or opacity. Use transform-based animations to avoid layout recalculation.

Signals to detect

• High per-shift impact fractions in RUM data.
• Frequent late DOM mutations in performance traces.
• Large visual changes recorded in session replay or synthetic filmstrip.

Key takeaway: Identify primary sources—images, embeds, fonts, and dynamic DOM changes—and target those sources for Cumulative Layout Shift (CLS) fixes.

Measurement & Benchmarks: Tools, Lab vs Field, Interpreting Results

CLS is measurable with both lab tools (Lighthouse, PageSpeed Insights) and field tools (Chrome User Experience Report, Web Vitals JS). Each method produces complementary insights; combine them to scope Cumulative Layout Shift (CLS) fixes effectively.

Lab tools

• Lighthouse: provides synthetic CLS scores, diagnostics, and trace-level visuals showing when shifts occur during simulated loads.
• PageSpeed Insights: shows both lab Lighthouse scores and CrUX field data for the origin.

Field tools

• Web Vitals JS: captures real user CLS events in production; use to compute distribution and identify high-percentile offenders.
• Chrome User Experience Report (CrUX): provides origin-level distributions across real sessions aggregated over 28 days.

How to run tests

1. Run Lighthouse on representative URLs to capture synthetic CLS and filmstrip evidence.
2. Instrument pages with Web Vitals JS to record CLS in production and collect session traces for problem sessions.
3. Use performance traces in DevTools to correlate DOM mutations, layout thrashing, and resource load times with CLS events.

Interpreting results

• Prioritize high-impact shifts (large impact fractions) even if they are rare.
• A high median or 75th-percentile CLS in CrUX indicates systemic issues across user bases or key pages.
• Compare lab filmstrips to RUM traces to reveal third-party-induced shifts occurring under specific conditions.

Key takeaway: Use synthetic tests to reproduce and debug; use RUM to verify real-world impact and to prioritize Cumulative Layout Shift (CLS) fixes by user impact.

Fixes by Category — Layout & Reserve Space

Reserve space in layout to prevent unexpected movement. The browser requires explicit constraints to allocate initial boxes; Cumulative Layout Shift (CLS) fixes in layout focus on defining dimensions, aspect ratios, and placeholder elements.

Set dimensions and aspect ratios

Include width and height attributes on <img>, <video>, and <iframe> elements or use CSS aspect-ratio to preserve intrinsic ratio. Use responsive units and maintain aspect containers to allow correct initial layout before media loads.

<img src="hero.jpg" width="1200" height="630" alt="..."/>
.container { aspect-ratio: 1200/630; }

Use CSS aspect-ratio and placeholder boxes

Define aspect-ratio for decorative containers and media wrappers. Implement placeholder background colors or skeletons that match final element dimensions to reduce perceptual shifts.

Avoid layout changes from client-side hydration

When using client-side rendering, render predictable initial markup or server-rendered placeholders that match final layout. Avoid replacing server-rendered elements with different dimensions during hydration.

Reserve space for dynamic UI

Allocate stable, consistent containers for banners, sticky elements, and interstitials. Preallocate heights for banners and preserve stacking context so late inserts do not push content unexpectedly.

Key takeaway: Apply explicit dimensions, aspect-ratio, and reserved containers to prevent layout reflows as a core set of Cumulative Layout Shift (CLS) fixes.

Fixes by Category — Media, Embeds & Ads

Images, videos, iframes, and ad slots are frequent high-impact sources of layout shifts. Apply targeted Cumulative Layout Shift (CLS) fixes that guarantee reserved space and predictable behavior for these media types.

Responsive images and srcset

Use srcset and sizes to deliver appropriate image dimensions while retaining width/height attributes or an aspect container. Precompute aspect ratios at build time to include correct attributes. See also Monthly Seo Services 2.

Lazy-loading with dimensions

Lazy-load offscreen images and videos using loading=”lazy” but ensure the element has reserved space before the resource loads. Apply placeholders sized by CSS or inline attributes to avoid reflow. See also Best Free Blogging Platform Publish Content Online.

Embeds and iframes

Wrap iframes in a container with defined dimensions or aspect-ratio. Use the loading attribute and set a placeholder or thumbnail. For embeds that can resize, use ResizeObserver to adjust only internal element layout without reflowing surrounding content.

Ad slots and third-party creatives

Allocate fixed ad-slot dimensions or flexible slots with maximum/minimum constraints. Use a stable slot height and design to accept creative variation while preventing inserts that change layout size. Apply a fallback element that renders when the creative is delayed or fails.

Platform examples

• WordPress: ensure featured images added by themes include width and height attributes and theme templates use aspect containers for responsive behavior.
• Shopify: set image size filters and include inline width/height or aspect-ratio in section templates to stabilize product listing layouts.

Key takeaway: Reserve space and provide predictable dimensions for all media and third-party embeds to eliminate the most common high-impact layout shifts as part of Cumulative Layout Shift (CLS) fixes.

Typography & Font Loading: Preventing Shifts Due to Fonts

Font loading affects layout when fallback fonts differ in metrics from final fonts. Cumulative Layout Shift (CLS) fixes for typography focus on reducing FOIT/FOUC and ensuring consistent metrics between fallback and final fonts.

Use font-display strategies

Set font-display to swap or optional for web fonts to avoid invisible text during load. Preload critical fonts with <link rel="preload" as="font" for key UI elements to reduce late metric changes.

Match fallback metrics

Select fallback fonts with similar x-height and character widths or use CSS font metrics overrides where supported. Variable fonts can reduce the number of separate files needed and lower load time variability.

Font subset and delivery

Deliver character subsets for languages and reduce weight files to lower latency. Host fonts on the same origin when possible or use high-performance CDNs to reduce cross-origin delays that trigger late swaps.

Testing font-related shifts

Use RUM to identify shifts corresponding to font load events; capture font timing with the Font Loading API and tie layout-shift events to specific font swaps in traces.

Key takeaway: Preload critical fonts, use font-display strategies, and align fallback metrics to eliminate font-induced layout shifts as targeted Cumulative Layout Shift (CLS) fixes.

Code & Performance — CSS, JS and Third-Party Scripts

Cumulative Layout Shift (CLS) fixes require controlling CSS and JavaScript behavior that changes layout after initial render. Optimize critical rendering path and limit scripts that mutate DOM or inject content asynchronously.

Critical CSS and render blocking

Inline critical CSS for above-the-fold content to present the stable shell quickly. Defer non-critical styles using media attributes or dynamic loading to avoid late style recalculations that shift layout.

Defer and async JavaScript

Load non-essential scripts with defer or async, and execute DOM mutations only after placeholders are present. Prefer progressive enhancement: render a usable layout first, then enhance behavior without changing dimensions.

Implement mutation containment

Contain DOM mutations within preallocated containers. Use position: absolute elements layered over stable flow content when showing transient elements such as tooltips, avoiding layout recalculation of parent flow.

Third-party script governance

Audit third-party scripts and restrict those that inject content or print banners. Load third-party scripts in iframes or sandboxes, or apply timeouts and fallbacks that prevent indefinite size changes. Monitor third-party-induced shifts via RUM and block or isolate offenders.

Use performance APIs

Employ PerformanceObserver for layout-shift entries to correlate shift timestamps with script execution. Automate detection by logging shift events and stack traces to locate responsible scripts.

Key takeaway: Control critical rendering path, contain DOM mutations, and govern third-party scripts to prevent unexpected post-render layout changes as central Cumulative Layout Shift (CLS) fixes.

Platform-Specific Guidance: WordPress, Shopify, Next.js

Each platform has unique templating, plugin, and rendering patterns. Apply Cumulative Layout Shift (CLS) fixes tailored to platform constraints and common misuse patterns.

WordPress

WordPress themes often control image output and dynamic widgets. Ensure theme templates output width/height attributes on images and set aspect-ratio wrappers. Audit plugins that inject banners or ads and configure them to reserve slot space. Use server-side rendering or precomputed thumbnails for consistent markup.

Shopify

Shopify sections and liquid templates should include image dimension data from product image objects and use aspect containers for collection pages. For apps that inject scripts, configure app placeholders or static fallbacks. Implement theme-level constraints for banners and cart drawers to prevent push content.

Next.js

Next.js provides Image component features that emit width/height and compute responsive sizes; use the component to ensure images reserve space. For hybrid rendering, stabilize server-rendered output and match client hydration output to avoid metrics divergence. Use getStaticProps/getServerSideProps to render stable markup when possible.

Common platform actions

• Audit plugins/apps and remove or configure those causing late injections.
• Implement theme-level aspect-ratio helpers and skeleton components that match final content dimensions.
• Use CDN and asset optimization to reduce load latency for media and fonts.

Key takeaway: Apply platform-specific templating and plugin configuration to ensure Cumulative Layout Shift (CLS) fixes integrate with CMS rendering models and third-party extensions.

Testing, Budgets & Maintenance: Monitoring CLS Over Time

Establish a monitoring strategy and enforce CLS performance budgets to prevent regressions. Combine continuous synthetic checks with Real User Monitoring to maintain acceptable Cumulative Layout Shift (CLS) levels.

Define a CLS budget

Set page-level CLS budgets by page type and critical conversion path. For example, allocate a maximum of 0.05 CLS for product detail pages and 0.10 for article pages, and enforce budgets in PR checks.

CI/CD integration

Integrate Lighthouse or a headless Chromium run into CI to catch regressions on pull requests. Configure thresholds to fail builds when synthetic CLS exceeds budget. Include image dimension checks and linting for templates in build steps.

RUM and alerting

Instrument Web Vitals JS to collect CLS events and send aggregated metrics to monitoring systems. Set alerting on percentiles (e.g., 75th or 95th) and monitor changes after releases or third-party updates.

Regression testing and release process

Include filmstrip comparisons for high-priority pages during release candidates. Maintain a regression test page that intentionally stresses common shift sources to validate fixes across devices and form factors.

Key takeaway: Formalize CLS budgets, integrate tests into CI, and maintain RUM-based monitoring to make Cumulative Layout Shift (CLS) fixes persistent and trackable.

Case Studies: Two Real-World CLS Fix Implementations

Case Study 1: E‑commerce product listing page

Background: An e‑commerce site reported a median CLS of 0.28 on product listing pages with high mobile traffic and a conversion drop on mobile devices.

Challenge: Dynamic promotional banners and product images without dimensions produced large-scale shifts when ad creatives loaded and when image lazy-load triggered without reserved space.

Fix implemented:

• Added explicit width and height attributes for all catalog images via build-time image metadata extraction.
• Wrapped promotional banners in reserved containers with a fixed height and close icon rendered as absolutely positioned content.
• Configured ad slots with predefined slot sizes and fallbacks to avoid late resize.

Results:

• Median CLS dropped from 0.28 to 0.06 within two weeks of deployment.
• Mobile add-to-cart rate improved by 7% on tested cohorts.
• Search visibility improved for category pages as Core Web Vitals classification moved to “Good.”

Takeaways: Prioritize image dimensioning and stable ad-slot design; measurable CLS reductions can yield conversion and SEO gains.

Case Study 2: News publisher with variable inline embeds

Background: A high-traffic news publisher experienced intermittent layout shifts from embedded content providers and varying ad creatives, with a reported 75th-percentile CLS of 0.18.

Challenge: Multiple third-party script vendors injected content asynchronously and replaced placeholders with content of unknown height.

Fix implemented:

• Introduced deterministic embed placeholders sized using metadata provided by embed providers when available.
• Deferred non-essential third-party scripts and sandboxed heavy creatives inside iframes with fixed dimensions.
• Implemented ResizeObserver within embeds to animate height changes within the iframe only, avoiding parent reflow.

Results:

• 75th-percentile CLS improved from 0.18 to 0.08 over four weeks.
• Decrease in accidental impressions and ad-related complaints from readers.
• Reduced support tickets related to layout breaks on articles with multiple embeds.

Takeaways: Embed vendors require contractual or technical controls; sandboxing and reserved placeholders are effective Cumulative Layout Shift (CLS) fixes for mixed third-party ecosystems.

Future Trends & Automation: Evolving Strategies for CLS

Browser features and automation trend toward proactive stability. Expect broader adoption of layout‑aware APIs and tools that assist with automatic reservation and precomputation of aspect ratios, which will influence Cumulative Layout Shift (CLS) fixes.

Emerging browser features

• Improved aspect-ratio support and layout containment features in browsers reduce the cost of reserving space.
• APIs that expose layout-shift attribution and better developer tooling to trace the source of shifts are becoming more robust.

Automation and build-time solutions

Automation at build time can extract media dimensions, compute aspect-ratio placeholders, and inject width/height attributes across templates. CI tasks can run visual diffing and validate that SSR markup matches client hydration to avoid hydration-induced shifts.

AI-assisted detection and remediation

Machine learning systems can detect patterns in RUM signals and recommend targeted fixes, such as predicting which elements should have fixed containers. Automated code transforms can insert aspect containers and preload critical assets during builds.

Policy and vendor management

Enterprises will adopt stricter third-party policies requiring vendors to provide embed metadata or offer stable-slot modes to support CLS budgets. Contractual SLAs for performance will include Core Web Vitals clauses.

Key takeaway: Invest in build-time automation, modern CSS features, and vendor governance to make Cumulative Layout Shift (CLS) fixes scalable and future-proof.

Frequently Asked Questions

Conclusion

Cumulative Layout Shift (CLS) fixes form a clear, implementable discipline: identify the primary sources of layout instability, apply deterministic layout constraints, and integrate testing and monitoring to prevent regressions. Begin by auditing the highest-impact pages with RUM and Lighthouse, then prioritize fixes that reserve space for images, media, ads, and dynamic UI elements. Implement font strategies that reduce metric changes, contain DOM mutations, and govern third-party scripts through sandboxing and fallbacks. For platform-specific work, update templates and configure plugins or apps to emit stable markup; Next.js sites should use framework image components and consistent SSR/CSR output. Enforce CLS performance budgets in CI and track percentiles in production telemetry to maintain improvements. The two case studies demonstrate measurable CLS reductions and correlated conversion and UX benefits when prioritized fixes target images, ad slots, and embed behavior. Operationalize Cumulative Layout Shift (CLS) fixes by adding automated build-time checks for image dimensions, running regular Lighthouse CI checks, and collecting Web Vitals in RUM. The next step is to run a focused CLS audit on priority pages, implement reserved-space fixes in the highest-impact components, and deploy monitoring to validate real-user improvement. Consistent application of these practices yields stable pages, improved Core Web Vitals, and measurable gains in user engagement and search visibility.