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Detailed CLS (Advanced Analysis)

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Definition

In-depth analysis of Cumulative Layout Shift that identifies each individual visual shift, its source, and its impact on the overall score.

Detailed CLS analysis goes beyond the simple global CLS score to precisely identify each layout shift event, its cause, and its contribution to the total score. Each Layout Shift is composed of two metrics: Impact Fraction (the proportion of the viewport affected) and Distance Fraction (the distance of the displacement). The browser's LayoutShift API allows capturing each event individually. The most frequent causes are images without explicit dimensions, dynamic ads, FOIT/FOUT fonts, dynamically injected content, and iframes without dimensions. Chrome DevTools and web.dev's Layout Shift Debugger let you visualize each shift in real time. The CLS metric was updated in 2021 to use a maximum session window, making detailed analysis even more important to understand the final score.

Advanced CLS Detailed Layout Shift CLS Deep Dive Debug CLS

Key Points

  • Breaks down the CLS score into individual events with Impact Fraction and Distance Fraction
  • Identifies the exact causes of each shift (images, ads, fonts, JS injection)
  • LayoutShift API and Chrome DevTools provide real-time debugging tools

Practical Examples

Diagnosing a high CLS

Detailed analysis reveals that 80% of CLS comes from an ad banner loaded 2 seconds after content, pushing all text downward. A CSS min-height reservation eliminates the shift.

Real-time debugging

Using Chrome DevTools Performance overlay, a developer identifies 5 distinct Layout Shifts caused by asynchronous loading of custom fonts. Adding font-display:optional eliminates 4 of the 5 shifts.

Frequently Asked Questions

In Chrome DevTools, open the Performance tab, start a recording, then reload the page. Layout Shifts appear as pink events in the timeline. Click on each one to see the affected element and the shift magnitude.

Google recommends a CLS below 0.1 for a good user experience. Between 0.1 and 0.25 needs improvement, and above 0.25 is considered poor. Detailed analysis helps identify which elements contribute most to the score.

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Last updated: 2026-02-07