By Shad Super in Questions —

What Is the Ideal Threshold for Cumulative Layout Shift (CLS), and How Can I Ensure Consistent Layout Stability Across All Devices?

Summary

The ideal Cumulative Layout Shift (CLS) score should be less than 0.1. CLS tracks visual stability, signifying the amount web page content unexpectedly shifts during load. To maintain consistency across all devices, it's vital to size all page elements correctly, avoid inserting content above already rendered elements and use transformation animations so they don't trigger layout changes. Let's dive deeper into understanding the CLS and ways to optimize it.

Cumulative Layout Shift: The Ideal Threshold

The Cumulative Layout Shift (CLS) metric is used to measure the visual stability of a web page, quantifying how frequently users experience unexpected layout shifts. The ideal CLS score for a web page should be less than 0.1 [CLS Web.dev, 2020].

Promoting Layout Stability

Proper Element Sizing

The size attributes for all media (like images, videos, GIFs, infographics, etc.) should be specified in the HTML or CSS. This helps the browser allocate correct space while the page is loading thereby minimizing unexpected shifts. Unspecified sizes are one of the most common causes of layout shifts [Optimize CLS, 2020].

Avoid Content Insertion Above Rendered Elements

Inserting new content above existing ones can push down the rendered content, causing layout shifting. To avoid this, aim to append new content below the current viewport or use a UI pattern like "Load More" rather than infinite scroll methods [Cumulative Layout Shift, 2020].

Using Transform Animations

Animations can also cause layout shifts if not handled properly. It's recommended to use transform CSS animations instead of positional animations, as they don't trigger any layout changes and offer a smoother user experience [Animations Update, 2017].

Preload Web Fonts

If web fonts are loaded late during the page load, they can cause Flash of Unstyled Text (FOUT) or Flash of Invisible Text (FOIT), both leading to layout shifts. Preloading key web fonts can help to avoid this [Optimize WebFont Loading, 2021].

Conclusion

To maintain a low CLS and thereby higher visual stability across all devices, it's necessary to set correct sizes for all page elements, avoid content insertion at the top of the page, use transform animations, and preload web fonts. Consistency in layout allows for a smoother user experience and better performance.

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