Why Cumulative Layout Shift (CLS) Can Hurt User Experience.pdf

Cumulative Layout Shift (CLS) is more than a technical metric - it’s a reflection of your website’s trustworthiness and usability. High CLS scores disrupt user interactions, cause accidental clicks, and lower search rankings. Learn how to identify, measure, and minimize CLS to deliver a stable, user-friendly experience that enhances engagement, accessibility, and SEO performance.

1. Why Cumulative Layout Shift (CLS) Can
Hurt User Experience

User experience (UX) has become the central factor in determining a website’s success. In
today’s digital age, visitors expect websites to be fast, responsive, and visually stable.

Anything less can frustrate users, increase bounce rates, and reduce conversions.
One of the most important yet often overlooked metrics that directly impacts user
experience is Cumulative Layout Shift (CLS).
CLS is one of Google’s Core Web Vitals, introduced in 2020 as part of a broader effort to
prioritize user-centric performance metrics.
Unlike traditional performance metrics that focus on page load speed, CLS measures
visual stability, specifically how much visible elements shift during page load or while
interacting with the page.
Imagine reading an article online. You’re about to click a “Read More” link, and suddenly
the text moves down because an image or advertisement has loaded above it.

2. This causes you to mis click and leads to frustration. These seemingly small shifts can
have a significant impact on the overall user experience.
Maintaining a low CLS score is essential for keeping users engaged, improving
accessibility, and boosting search engine rankings.
Understanding Cumulative Layout Shift (CLS)
Cumulative Layout Shift measures the unexpected movement of visible elements in the
viewport.
Unlike other web performance metrics like page load time or interactivity, CLS focuses
purely on visual stability, which is critical for a positive user experience.
How CLS Is Calculated
CLS is calculated using two primary factors:
1. Impact Fraction - The percentage of the viewport affected by the shift.
2. Distance Fraction: How far the element moves relative to the viewport.
The final CLS score is the sum of all layout shifts that occur throughout the page’s
lifecycle. Google recommends a CLS score of 0.1 or less as good, while a score above
0.25 is considered poor.
Example:
Suppose you visit a news website. A headline is visible on your screen, and you start
reading. Suddenly, an image above the headline loads, pushing the text down. This
movement contributes to the CLS score. If multiple images, ads, or dynamic content shift
during the session, the CLS score accumulates.
CLS vs Other Core Web Vitals
To fully understand CLS, it’s important to compare it with other Core Web Vitals:
• Largest Content ful Paint (LCP)-Measures loading performance.
• First Input Delay (FID)-Measures interactivity and responsiveness.
• Cumulative Layout Shift (CLS)- Measures visual stability.

3. While LCP and FID focus on speed and responsiveness, CLS ensures the page remains
predictable and stable for users.
Why CLS Matters for User Experience
High CLS scores can significantly impact user engagement, satisfaction, and retention.
Let’s explore why:
1. Disrupted Reading and Interaction
Unexpected layout shifts disrupt the user’s reading flow. Even small jumps can be irritating
when reading long-form content or product descriptions. On e-commerce sites, layout
shifts can even lead to abandoned carts if users are unable to click buttons accurately.
2. Accidental Clicks
When elements move unexpectedly, users may click the wrong buttons or links. For
example:
• Clicking an advertisement instead of the submit button.
• Triggering navigation away from the page due to mis clicks.
Accidental clicks frustrate users and can reduce trust and engagement, especially for
sites handling sensitive actions like payments or subscriptions.
3. Negative Impact on Accessibility
Users with disabilities rely on predictable layouts for navigation. Frequent layout shifts
can interfere with:
• Screen readers
• Keyboard navigation
• Voice control accessibility
High CLS can make a site less inclusive, which may affect your legal compliance and
overall reputation.

4. 4. Decreased User Trust
A visually unstable site often appears unprofessional or unreliable. Visitors may
associate layout issues with poor website quality or even security concerns, which
reduces trust in your brand.
5. Higher Bounce Rates
Studies show that users are more likely to leave a website that feels unstable or
unpredictable. This increases bounce rates, reduces average session duration, and
negatively impacts SEO performance.
6. Mobile vs. Desktop
Mobile users are particularly affected by CLS because screen space is smaller. Ads,
images, or dynamic content can shift elements more drastically on mobile, making CLS
management even more critical for mobile-first indexing and user retention.
SEO Implications of High CLS
CLS is a ranking factor in Google’s search algorithm. High CLS can:
• Reduce search engine rankings
• Lower click-through rates (CTR)
• Negatively impact dwell time and conversions
Websites with low CLS provide a better user experience, which Google interprets as a
sign of quality. By improving CLS, you can simultaneously enhance UX and SEO
performance.
Common Causes of High CLS
High CLS often results from poor design or technical implementation. Here are the most
common causes:

5. 1. Images Without Dimensions
Images without defined width and height attributes force the browser to calculate layout
on the fly, leading to content shifts.
Solution: Always define image dimensions in HTML or CSS. Use aspect ratio to ensure
responsiveness without layout shifts.
2. Ads, Embeds, and I Frames
Third-party content often loads asynchronously and resizes dynamically. Without reserved
space, it can push content down unexpectedly.
Solution: Predefine containers for ads, videos, and embeds. For responsive designs, use
minimum height placeholders.
3. Dynamically Injected Content
Pop-ups, banners, notifications, or chat widgets that appear after the initial load can push
content down, creating CLS.
Solution: Only inject content above existing elements in response to user interaction.
4. Web Fonts
Web fonts that load after fallback fonts can cause text reflow. Even minor differences in
font weight or size can trigger layout shifts.
Solution: Use font-display: swap or preload key fonts to minimize shifts.
5. Animations and Transitions
Animations affecting layout properties (like width, height, margin, and padding) can
trigger CLS.
Solution: Animate only transform or opacity properties for smooth motion without layout
shifts.

6. Best Practices to Reduce CLS
Reducing CLS requires deliberate strategies, both technical and design-oriented.
1. Specify Image and Video Dimensions
• Define width and height for all media elements.
• Use CSS aspect-ratio for responsive designs.
• Preload critical images to prevent jumps.
2. Reserve Space for Ads and Embeds
• Assign fixed container sizes for dynamic elements.
• Avoid dynamically inserting content above visible content.
• For responsive layouts, use flexible placeholders to maintain stability.
3. Avoid Content Shifts Above Existing Content
• Ensure banners, pop-ups, and notifications appear below or alongside content
unless triggered by user interaction.
• For critical alerts, reserve space at the top to prevent shifting.
4. Font Loading Strategies
• Use font-display: swap to display fallback fonts until the custom font is ready.
• Preload critical fonts for faster rendering.
• Minimize differences in size between fallback and custom fonts.
5. Test and Monitor Regularly
• Use Google Page Speed Insights, Chrome Dev Tools, or the Web Vitals
Extension to identify layout shifts.
• Test both desktop and mobile versions.
• Monitor changes after each content update, plugin installation, or design change.

7. Tools to Measure CLS
1. Google Page Speed Insights
Provides both lab data and real-world metrics for CLS. It highlights specific elements
causing layout shifts.
2. Chrome Dev Tools
Use the Performance tab to record page load and visualize CLS events. This helps
pinpoint the exact element causing the shift.
3. Web Vitals Extension
Browser extension that measures real-time CLS scores across browsing sessions, helping
track improvements after fixes.
Additional Tips for Maintaining Low CLS
• Use skeleton screens: Placeholder elements give users visual feedback while
content loads.
• Prioritize critical content: Load main content first to reduce visible shifts.
• Audit third-party scripts: Ads, social embeds, or widgets often cause CLS;
optimize or defer them.
• Minimize dynamic content above the fold: Anything loaded above visible content
can disrupt stability.
• Regularly review site updates: Even minor updates can impact layout shifts.
Conclusion
Cumulative Layout Shift is more than a metric-it is a measure of user trust and
satisfaction. High CLS frustrates users, reduces conversions, and negatively affects SEO.
By understanding its causes and implementing best practices, website owners can:
• Improve user engagement and retention
• Enhance accessibility and inclusivity
• Boost search engine rankings

8. • Increase conversion rates
Maintaining low CLS requires continuous monitoring, testing, and optimization, but the
benefits are substantial. A visually stable website ensures a smooth, predictable, and
enjoyable user experience, which ultimately drives success in today’s competitive online
landscape.
FAQ
Q1: What is a good CLS score?
A1: A CLS score of 0.1 or less is considered good, indicating minimal unexpected layout
shifts.
Q2: How can I improve my website’s CLS?
A2: Specify image and video dimensions, reserve space for dynamic elements, avoid
inserting content above existing elements, and implement proper font loading strategies.
Q3: Does CLS affect SEO rankings?
A3: Yes, Google includes CLS in its Core Web Vitals metrics, impacting both ranking and
visibility.
Q4: Can high CLS cause higher bounce rates?
A4: Yes, unstable layouts frustrate users, leading them to leave a site prematurely.
Q5: Are mobile users more affected by CLS?
A5: Absolutely. Smaller screens amplify layout shifts, making it crucial to optimize for
mobile first.
Q6: Do animations contribute to CLS?
A6: Only if they affect layout properties. Animations on transform or opacity do not count
toward CLS.
Q7: What tools help monitor CLS?
A7: Google PageSpeed Insights, Chrome Dev Tools, Web Vitals Extension, and Core Web
Vitals reports in Google Search Console.
Q8: How often should I test CLS?
A8: Regularly-after major site updates, design changes, or content additions.

9. Q9: Do ads always increase CLS?
A9: Not always. Ads can be optimized by reserving fixed space for them.
Q10: Can web fonts cause layout shifts?
A10: Yes. Using proper font-loading strategies prevents text reflow and improves CLS.