Generated using Layout Shift GIF Generator. Ignoring them means the browser may not be able to reserve sufficient space in advance of them loading. The most common causes of CLS include images without dimensions (see below), which can push down content when they load and snap into place. Cumulative Layout Shift (CLS) measures the instability of content. 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. While great for looking at opportunities to improve your user-experience, always try to consult real-world data for a complete picture of what actual users are seeing. Keep in mind that Lighthouse is a lab tool. You can also find Lighthouse in PageSpeed Insights, CI, and WebPageTest. You can find it in the Chrome DevTools suite of debugging tools and run it against any web page, whether public or requiring authentication. Lighthouse is an open-source, automated tool for improving the quality of web pages. In this guide, we will be using Lighthouse to identify opportunities to improve the Core Web Vitals, walking through optimizations for each metric. Above, the Stack Overflow “The Key” hero image was the Largest Contentful Paint element Images can impact the CWV in a number of ways. CWV is part of a set of page experience signals Google Search will be evaluating for ranking. It’s an initiative by Google to share unified guidance for quality signals that can be key to delivering a great user-experience on the web. You may have heard of Core Web Vitals (CWV). Image impact on user-experience and the Core Web Vitals If using 11ty for your static sites, try the 11ty high-performance blog template. You can of course also do this manually just using the element directly. Note: Modern image components that build on, like Next.js (for React) and Nuxt image (for Vue) try to bake in as many of these concepts as possible by default. While not render-blocking, they can indirectly impact render performance. Avoid images causing network contention with other critical resources like CSS and JS.Use CSS aspect-ratio or aspect ratio boxes to reserve space otherwise.Set dimensions (width, height) on your images.Lazy-load offscreen images (reduce network contention for key resources).Avoid wasting pixels (compress, don’t serve overly high DPR images).Use srcset + efficient modern image formats.Given how central it is to image optimization on the web, let’s catch up on what it can do and how it can help improve user-experience and the Core Web Vitals.įirst, some tips to get us started optimizing our metrics: The humble element has gained some superpowers since it was created. We like images over on the Stack Overflow blog too. Images are so prominent that they are part of the most important content in over ~70% of pages on both mobile and desktop according to the largest contentful paint metric. This isn’t a huge surprise as we humans are quite visual and the tag has been around for almost 30 years.

Images are one of the most pervasive parts of the web.

Android image gallery stack