The Almost-Complete Guide to Cumulative Layout Shift

Here’s Jess B. Peck writing all about Google’s Core Web Vitals:

Let’s step back one. CLS is when you’re about to click on a link, and the whole page shifts and you click on a different link instead. It’s when you’re halfway through a blogpost and an ad loads and you lose your place. It is when… the layout shifts. At least, that’s what it’s trying to measure– both those shifts, how often they happen, and the irritation that causes the user.

I didn’t quite understand just how complex Cumulative Layout Shift is before reading Jess’s piece. As Jess explains:

CLS is a measure for a robot to approximate the user perception of instability. This means we’re getting a unit of change over time. It’s a three dimensional equation, and there are tons of things that can affect it. […] The idea is more to alert devs to a problem area, rather than be a perfect measurement of how annoying a page is.

I had this problem on, of all places, Google dot com. I kept tapping an element just as it appeared on screen and this sent me to the wrong page.

Jess notes that these metrics are sometimes more of an art than a science, and so we shouldn’t be obsessed with making sure that just these Core Web Vital metrics are okay. Chris mentioned a while ago that he worries folks might begin to game these metrics for improving their SEO:

This feels like the start of a weird new era of web performance where the metrics of web performance have shifted to user-centric measurements, but people are implementing tricky strategies to game those numbers with methods that, if anything, slightly harm user experience.

Harry Roberts mentioned something similar:

I feel like this is our responsibility as web developers, to explain that what we want to do here is reduce user misery on our websites. That’s not to say it’s easy, though, and there’s certainly not much we can do to avoid the shady folks who’ll game these metrics only to improve SEO.

As Jeremy wrote just the other day:

The map is not the territory. The numbers are a proxy for user experience, but it’s notoriously difficult to measure intangible ideas like pain and frustration.

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Deliver Enhanced Media Experiences With Google’s Core Web Vitals

Hello! Satarupa Chatterjee from Cloudinary. There is a big change coming from Google in May 2021 having to do with their Core Web Vitals (CWVs). It’s worth paying attention here, as this is going to be a SEO factor.

I recently spoke with Tamas Piros about CWVs. The May 2021 update will factor in CWVs, along with other factors like mobile-friendliness and safe browsing, to generate a set of benchmarks for search rankings. Doubtless, the CWVs will directly affect traffic for websites and apps alike. Tamas is a developer-experience engineer at Cloudinary, a media-optimization expert, and a Google developer-expert in web technologies and web performance.

Here’s a written version of the video above, where the questions (Qs) are me, Satarupa, asking and Tamas answering (As).

Q: How did Google arrive at the three Core Web Vitals and their values?

A: As a dominant force in the search space, Google has researched in depth what constitutes a superb user experience, arriving at three important factors, which the company calls, collectively, the Core Web Vitals.

Before explaining them, I’d like to recommend an informative article, published last May on the Chromium Blog, titled The Science Behind Web Vitals. At the bottom of the piece are links to papers on the research that led to the guidelines for accurately evaluating user experiences.

Now back to the three Core Web Vitals. The first one affects page-load speed, which Google calls Largest Contentful Paint (LCP) with a recommendation of 2.5 seconds or less for the largest element on a page to load.

The second metric is First Input Delay (FID), which is a delta between a user trying to interact with a page, and the browser effectively executing that action. Google recommends 100 milliseconds or less. 

The third and last metric is Cumulative Layout Shift (CLS), which measures how stable a site is while it’s loading or while you’re interacting with it. In other words it is a measurement of individual layout shifts as well as unexpected layout shifts that happen during the lifespan of a page. The calculation involves impact and distance fractions which are multiplied together to give a final value. Google advocates this value to be 0.1 or less.

Q: How do the Core Web Vitals affect e-commerce?

A: Behind the ranking of Google search results are many factors, such as whether you use HTTPS and how you structure your content. Let’s not forget that relevant and well-presented content is as important as excellent page performance. The difference that core web vitals will make cannot be overstated. Google returns multiple suggestions for every search, however remember that good relevance is going to take priority. In other words good page experience will not override having great relevant content For example, if you search for Cloudinary, Google will likely show the Cloudinary site at the top of the results page. Page experience will become relevant when there are multiple available results, for a more generic search such as ‘best sports car’. In this case Google establishes that ranking based on the page’s user experience, too, which is determined by the Core Web Vitals.

Q: What about the other web vitals, such as the Lighthouse metrics? Do they still matter?

A: Businesses should focus primarily on meeting or staying below the threshold of the Core Web Vitals. However, they must also keep in mind that their page load times could be affected by other metrics, such as the length of time the first purchase takes and the first contentful paint.

For example, to find out what contributes to a bad First Input Delay—the FID, check the total blocking time and time to interact. Those are also vitals, just not part of the Core Web Vitals. You can also customize metrics with the many robust APIs from Google.. Such metrics could prove to be invaluable in helping you identify and resolve performance issues.

Q: Let’s talk about the Largest Contentful Paint metric, called LCP. Typically, the heaviest element on a webpage or in an app is an image. How would you reduce LCP and keep it below the Google threshold of 2.5 seconds?

A: What’s important to remember with regards to LCP is that we are talking about the largest piece of content that gets loaded on a page, and that is visible in the viewport (that is, it’s visible above the fold). Due to popular UX design patterns it’s likely that the largest, visible element is a hero image.

Google watches for <img> elements as well as <image> elements inside an SVG element. Video elements are considered too but only if they contain a poster attribute. Also of importance to Google are block-level elements, such as text-related ones like <h1>, <h2>, etc., and <span>.

All that means that you must load the largest piece of content as fast as possible. If your LCP is a hero image, be sure to optimize it—but without degrading the visual effects. Check out Cloudinary’s myriad effective and intuitive options for optimization. If you can strike a good balance between the file size and the visual fidelity of your image, your LCP will shine. 

Q: Suppose it’s now May 2021. What’s the likely effect of Google’s new criteria for search rankings for an e-commerce business that has surpassed the thresholds of all three or a couple of the Core Web Vitals?

A: According to Google, sites that meet the thresholds of the Core Web Vitals enjoy a 24-percent lower abandonment rate. The more you adhere to Google’s guidelines, the more engaging your site or app becomes and the faster your sales will grow. Needless to say, an appealing user experience attracts visitors and retains them, winning you an edge over the competition. Of course bear in mind the other search optimization guidelines set out by Google.

Again, be sure to optimize images, especially the most sizable one in the viewport, so that they load as fast as possible.

Q:  It sounds like e-commerce businesses should immediately start exploring ways to meet or better the vitals’ limits. Before we wrap up, what does the future look like for Core Web Vitals?

A: Late last year, Google held a conference and there were multiple talks touching upon this exact subject. All major changes will go into effect on a per-year basis, and Google has committed to announcing them well in advance.

Behind the scenes, Google is constantly collecting data from the field and checking them against user expectations. The first contentful paint, which I mentioned before, is under consideration as another Core Web Vital. Also, Google is thinking about reducing the yardstick for the First Input Delay metric—the FID, remember?—from 100 milliseconds to 75 or even 50.

Beyond that, Google has received a lot of feedback about some of the Core Web Vitals not working well for single-page apps. That’s because those apps are loaded only once. Even if they score an ideal Cumulative Layout Shift—that’s CLS—as you click around the page, things might move around and bring down the score. Down the road, Google might modify CLS to better accommodate single-page apps. 

Also on Google’s radar screen are metrics for security, privacy, and accessibility. Google promises to fine-tune the current metrics and launch new ones more frequently than major releases, including the introduction of new Core Web Vital metrics. 

So, change is the only constant here. I see a bright future for the vitals and have no doubt that we’re in good hands. Remember that Google vigilantly collects real user data as analytics to help figure out the appropriate standards. As long as you keep up with the developments and ensure that your site or app comply with the rules, you’ll get all greens throughout the scoreboard. That’s a great spot to be in.

Cloudinary offers myriad resources on media experience (MX), notably the MX Matters podcast, which encompasses experts’ take on the trends in today’s visual economy along with bulletins on new products and enhancements. Do check them out.


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The Core Web Vitals hype train

Some baby bear thinking from Katie Sylor-Miller:

my excitement for Core Web Vitals is tempered with a healthy skepticism. I’m not yet convinced that Largest Contentful Paint (LCP), First Input Delay (FID), and Cumulative Layout Shift (CLS) are the right metrics that all sites should be measuring themselves against. I worry that the outsized emphasis placed on Core Web Vitals by including them in SEO scoring will result in developers focusing solely on those three numbers without truly understanding what they mean and, more importantly, what they don’t mean.

Katie is pro-Web Core Vitals because of their correlation with real user experiences, but there is a lot more to think about. If we focus only on these metrics (because we have now an extremely strong incentive to do so) we’re missing out on a lot. They may not be the analytics that matter most to us or that correlate with business goals.

The horse’s mouth says the SEO implications don’t start until May 2021.

I admit I’ve been on the hype train myself a little bit. I like all of Katie’s points but I think I’ll still call it a step forward for web analytics. Robin also mentioned Sentry could do the tracking the other day.

Jeremy mentions the proliferation of initials:

Personally, my beef with core web vitals is that they introduce even more uneccessary initialisms (see, for example, Harry’s recent post where he uses CWV metrics like LCP, FID, and CLS—alongside TTFB and SI—to look at PLPs, PDPs, and SRPs. I mean, WTF?).

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