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After recording a new trace in the Performance panel, the Experience section of the results is populated with a red-tinted bar displaying a Layout Shift record. This is particularly helpful to get more detail on load CLS issues since this is easily replicated with a reload performance profile. The Summary view for a Layout Shift record includes the cumulative layout shift score as well as a rectangle overlay showing the affected regions. The Performance panel in DevTools also highlights layout shifts in the Experience section. However, the shifted element should be sufficient to help you identify and resolve the root cause. For example, if a new element is inserted into the DOM, the elements that are beneath it will show in this audit, but the root cause is the addition of the new element above. Lighthouse will identify the elements that were shifted, but often these are the ones impacted rather than the elements causing the CLS. If you see a difference between these, this is likely caused by post-load CLS. PageSpeed Insights will show both the user-perceived CLS from a URL where it exists in the 'Discover what your real users are experiencing' section, and also the lab-based load CLS in the 'Diagnose performance issues' section beneath. Other common causes of post-load CLS are on interactions of transitions, for example on Single Page Apps, which take longer than the 500 millisecond grace period. However, other post-load shifts that are unexpected by the user may be included where there was no qualifying interaction-for example, if you scroll down the page and lazy-loaded content is loaded and that causes shifts. Many post-load shifts may occur as the result of a user interaction and therefore will be excluded from the CLS score as they are expected shifts-as long as they occur within 500 milliseconds of that interaction. Layout shifts are very common during page load, as all the necessary resources are fetched to initially render the page, but layout shifts can also happen after the initial load.

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Web performance lab tools like Lighthouse may not show the full CLS of a page as they typically do a simple load of the page to measure some web performance metrics and provide some guidance (though Lighthouse user flows do allow you measure beyond the default page load audit).ĬrUX is the official dataset of the Web Vitals program, and for that, CLS is measured throughout the full life of the page and not just during the initial page load that lab tools typically measure. It is quite common to hear developers think the CLS measured by the Chrome UX Report (CrUX) is incorrect as it does not match the CLS they measure using Chrome DevTools or other lab tools. A big part of the problem is understanding your CLS score-the fix afterwards is often the easier part! CLS in lab tools versus field # Understanding where your shifts are coming from #īefore we start looking at solutions to common CLS issues, it's always important to understand your CLS score and where the shifts are coming from.

Dynamically injected content such as ads, embeds, and iframes without dimensions.

Ads, embeds, and iframes without dimensions.

The most common causes of a poor CLS are: In this guide, we'll cover optimizing common causes of layout shifts. Unlike the other Core Web Vitals, which are time-based values measured in seconds or milliseconds, the CLS score is a unitless value based on a calculation of how much content is shifting and by how far. To provide a good user experience, sites should strive to have a CLS of 0.1 or less for at least 75% of page visits. They're often caused when visible elements are forced to move because another element was suddenly added to the page or resized. Such experiences are visually jarring and frustrating. This is very common on the web, including when reading the news, or trying to click those 'Search' or 'Add to Cart' buttons. Imagine you've started reading an article when all of a sudden elements shift around the page, throwing you off and requiring you to find your place again.

Layout shifts can be distracting to users. It looks at how much visible content shifted in the viewport as well as the distance the elements impacted were shifted. 'I was about to click that! Why did it move? 😭'Ĭumulative Layout Shift (CLS) is one of the three Core Web Vitals metrics, and it measures the instability of content by summing shift scores across layout shifts that don't occur within 500 milliseconds of user input. Reduce CLS by ensuring pages are eligible for the bfcache.Ads, embeds, and other late-loaded content.Understanding where your shifts are coming from.

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