Cumulative Layout Shift (CLS) is a vital metric in the field of web performance and user experience. Understanding and optimizing for CLS can significantly enhance the usability and appeal of a website. Let's delve into what CLS is, why it matters, and how to improve it.
What is Cumulative Layout Shift (CLS)?
CLS is a metric that measures the stability of a webpage’s content as it loads. It's a part of Core Web Vitals, a set of metrics by Google that assess the user experience of web pages. In simpler terms, CLS quantifies how much the visible content on a page shifts around as the page fully loads. These shifts can be frustrating for users, as they might click on something unintentionally or lose their place while reading.
Why is CLS Important?
User Experience: High CLS can lead to a poor user experience. If elements on the page move unpredictably, it can confuse or frustrate users.
SEO Impact: Google uses Core Web Vitals, including CLS, as ranking factors for websites. A lower CLS can contribute to better search engine rankings.
Conversion Rates: Websites with low CLS often have higher conversion rates, as users are more likely to interact positively with stable and predictable pages.
Common Causes of High CLS
Images without dimensions: If images load without specified height and width, they can push content around as they load.
Ads, embeds, and iframes without dimensions: Similar to images, these elements can cause shifts if their dimensions aren’t defined.
Dynamically injected content: Content that loads dynamically, like pop-ups or banners, can cause shifts.
Web Fonts causing FOIT/FOUT: Flash of Invisible Text (FOIT) or Flash of Unstyled Text (FOUT) can also lead to layout shifts.
How to Improve CLS
Specify size attributes for images and videos: Always include width and height attributes for media. This helps the browser allocate space while the media is loading.
Ensure ad elements have a reserved space: Prevent ads from pushing content around by reserving sufficient space for them.
Avoid inserting new content above existing content: Be careful with dynamically injected content and try to add it in places that won’t shift existing content.
Preload web fonts: Use font-display: swap to minimize the impact of FOIT and FOUT.
Optimize CSS and JavaScript loading: Prioritize loading critical CSS and JavaScript to stabilize layout early.
Tools for Measuring and Monitoring CLS
Google PageSpeed Insights: Provides a detailed analysis of CLS and other Core Web Vitals.
Chrome DevTools: Offers a performance panel to visualize layout shifts.
Web Vitals Chrome Extension: Helps monitor Core Web Vitals, including CLS, in real-time.
Conclusion
Optimizing CLS is crucial for providing a pleasant user experience and improving a website's overall performance and SEO ranking. By understanding its causes and implementing strategies to minimize layout shifts, web developers can significantly enhance the stability and usability of their websites.