Stop Content From Jumping Around Your Pages

CLS tracks unexpected layout shifts that occur during page loading. High CLS scores indicate content jumps around as new elements load, creating jarring user experiences that damage engagement and conversions.

Understanding CLS Benchmarks

Good CLS: 0.1 or less
Needs Improvement: 0.1 to 0.25
Poor CLS: 0.25 or more

Unlike other Core Web Vitals measured in seconds, CLS uses a unitless score representing visual instability impact. Lower scores indicate more stable, predictable layouts.

How CLS Calculates Visual Chaos

CLS calculates based on impact fraction (how much content moved) multiplied by distance fraction (how far content moved). The score represents the cumulative effect of all unexpected shifts during page load.

Impact Fraction: Percentage of viewport affected by unstable elements
Distance Fraction: How far unstable elements moved relative to viewport

For example, if 50% of your page shifts down by 25% of the viewport height, your CLS score would be 0.125 (0.5 × 0.25). Multiple shifts accumulate throughout the loading process.

Common Layout Shift Culprits

Images Without Dimensions: Browsers cannot reserve space for images lacking width and height attributes. Content below shifts when images load.

Dynamic Ads: Advertisement slots that expand or change size push existing content around unexpectedly.

Web Fonts: Font swaps can change text dimensions, causing paragraphs to reflow and shift surrounding elements.

Injected Content: Elements inserted above existing content force everything below to move down.

Embeds and Widgets: Social media widgets, videos, and third-party content often load with unpredictable dimensions.

Immediate CLS Improvements

Target these high-impact fixes:

Set Explicit Image Dimensions: Add width and height attributes to every image. Modern browsers use these values to calculate aspect ratios and reserve appropriate space.

<img src="hero.jpg" width="800" height="400" alt="Hero image">

Reserve Space for Ads: Define container dimensions for advertisement slots before ads load. Use CSS to maintain consistent spacing.

Optimize Font Loading: Use font-display: swap to prevent invisible text periods while maintaining layout stability during font loading.

Avoid Content Injection: Never insert new elements above existing content. Append dynamic content to designated containers or use fixed-height placeholders.

Advanced Stabilization Techniques

Use CSS Aspect Ratio: For responsive images, combine explicit dimensions with CSS aspect-ratio property to maintain proportions across screen sizes.

Implement Skeleton Screens: Show placeholder content matching final layout dimensions while actual content loads.

Preload Critical Resources: Use <link rel="preload"> for fonts and above-fold images to reduce loading-related shifts.

Transform Instead of Changing Layout: Use CSS transforms and opacity changes for animations instead of modifying element dimensions or positions.

Measuring Layout Stability

Use Chrome DevTools Performance panel to visualize layout shifts in real-time. The Layout Instability API provides programmatic access to CLS measurements for continuous monitoring.

Track improvements across device types—mobile devices often experience worse CLS due to variable screen sizes and slower loading speeds.

Business Impact of Visual Stability

Poor CLS creates accidental clicks when buttons move just as users attempt to interact. E-commerce sites lose conversions when "Add to Cart" buttons shift during product page loading.

Content sites see increased bounce rates when articles become unreadable due to shifting text and images. Users abandon pages that feel broken or unprofessional.

Mobile users particularly suffer from layout shifts due to touch interfaces. Accidental taps on shifted elements frustrate users and damage trust.