A computer screen displaying a website experiencing a large layout shift

Understanding and Avoiding Large Layout Shifts

Web performance is a crucial aspect of user experience. One of the key elements that can significantly impact this experience is the occurrence of large layout shifts. But what exactly are these, and how can we avoid them? Let’s delve into this topic and explore the answers.

Defining Large Layout Shifts

Large Layout Shifts, often abbreviated as LLS, refer to the unexpected movement of content on a webpage. This usually happens when visible elements on a page change their position during rendering. It’s a phenomenon that can lead to a frustrating user experience, as it can cause users to lose their place on a page or accidentally click on the wrong thing.

Google uses a metric called Cumulative Layout Shift (CLS) to measure the instability of content. A high CLS score indicates a high level of layout shift, which is detrimental to the user experience. Therefore, it’s essential to understand and mitigate large layout shifts to improve your website’s performance and user satisfaction.

Causes of Large Layout Shifts

Images Without Dimensions

One of the most common causes of large layout shifts is images without specified dimensions. When an image doesn’t have a defined width and height, the browser doesn’t know how much space to allocate for it until it starts loading. This can cause other elements on the page to move around, resulting in a layout shift.

Similarly, dynamically loaded content, such as ads or iframes without reserved space, can also cause layout shifts. When these elements load, they can push down or displace existing content, leading to an unstable layout.

Web Fonts Causing FOIT/FOUT

Web fonts can also contribute to large layout shifts. When a web font loads, it can cause Flash of Invisible Text (FOIT) or Flash of Unstyled Text (FOUT). These phenomena occur when the browser displays a fallback font while the web font is loading, then swaps the fonts once the web font becomes available. This swap can cause a shift in the layout if the web font and the fallback font have different metrics.

Another related issue is the late-loading of icons. If you’re using a web font for icons, the icons might not appear or may display incorrectly until the font loads, causing a layout shift.

How to Avoid Large Layout Shifts

Specify Size Attributes for Images and Videos

One of the simplest ways to avoid large layout shifts is to always specify the width and height attributes for images and videos. This allows the browser to allocate the correct amount of space for these elements before they load, preventing them from pushing other content around.

For responsive images, you can use CSS aspect ratio boxes to maintain the correct aspect ratio regardless of the viewport size. This technique can help prevent layout shifts caused by images resizing.

Reserve Space for Ads and Embeds

Ads and embeds are often loaded dynamically, which can cause layout shifts. To prevent this, you can reserve space for these elements ahead of time. This can be done by setting a fixed size for the ad or embed container, or by using CSS aspect ratio boxes to reserve space based on the expected aspect ratio of the content.

It’s also a good idea to avoid inserting new content above existing content, as this can push down the existing content and cause a layout shift. Instead, try to insert new content below the fold or at the end of the content.

Optimize Font Loading

To avoid layout shifts caused by web fonts, you can optimize your font loading strategy. One approach is to use font-display: optional or font-display: swap in your @font-face style. This tells the browser to use a fallback font if the web font isn’t available, and to swap the fonts as soon as the web font loads.

Another strategy is to preload your web fonts. This tells the browser to start loading the fonts early, which can help ensure they’re available by the time they’re needed. However, be careful not to preload too many fonts, as this can slow down your page load time.


Large layout shifts can significantly impact the user experience on your website. By understanding the causes of these shifts and implementing strategies to avoid them, you can improve your site’s performance and keep your users happy. Remember, a stable layout isn’t just good for usability – it’s also a factor that search engines consider when ranking your site.

So, take the time to audit your site for potential layout shifts, and make the necessary adjustments. Your users – and your search engine rankings – will thank you.

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