Cumulative Layout Shift 101. What Is CLS In SEO?

Cumulative Layout Shift 101. What Is CLS In SEO?

Another year in SEO and another metric to monitor and optimise, to ensure your website is leading the way in search results. But, what is Cumulative Layout Shift? How can it affect SEO? And what can we do to improve it?

Well, let’s take a look.

Table of Contents

Core Web Vitals

Before we take a look at Cumulative Layout Shift, we need to take a quick look at Core Web Vitals.

Core Web Vitals are a set of metrics, developed by Google to measure the page loading performance of a website.

The reason? User experience. Google are on a quest to further improve user experience for its users. And they are consistently developing new ways to test and measure websites, in the interest of a better experience for website visitors, referred from Google.

Core Web Vitals will be rolled out within a core Google update in May 2021. Whilst it doesn’t seem that Google will penalise websites not meeting the target score for Core Web Vitals, it’s becoming clear that the websites that do meet these standards will be eligible for increased search ranking positions.

It is made clear by Google that these metrics will evolve over time, and are by no means perfect in determining a great user experience.

The weight of these new metrics will carry within the ranking algorithms is unknown, but is thought to be minimal in relation to known ranking factors, including quality of content and backlink profile.

What is Cumulative Layout Shift?

Cumulative Layout Shift is one of the main three, Core Web Vitals, used to help measure the quality of a website for user experience.

The metric measures visual stability of a web page.

Most of us have experienced elements on a web page, moving or shifting during loading. Perhaps you’re looking to click a ‘learn more’ button on a website home page, and then a picture loads above or below it, shifting the position of the button.

These unexpected shifts in layout are measured throughout the lifespan of a page, or user session. When an element in the page’s layout shifts position from one rendered frame to the next, a shift in position is recorded.

This is layout shift.

It goes without saying, that is annoying. It disrupts what the website user is trying to do, and they may just end up clicking another link instead which can be extremely frustrating when navigating a website.

The issue seems to be particularly prevalent on websites displaying adverts, as they load in various places on the page.

The Cumulative Layout Shift metric is designed to enable website owners or SEO agencies to measure the CLS and take action to improve it.

The current ways to measure the Cumulative Layout Shift of a web page are:

  • PageSpeed Insights – Use this tool for in-depth analysis on CLS and other Core Web Vital metrics. Along with suggested areas to improve.
  • Search Console – The dashboard now includes a Core Web Vitals report, showing whether your pages meet the standards, require improvement, or are poorly rated.
  • Chrome User Experience Report – A reporting tool designed to measure what Chrome users are experiencing when loading a web page.
Bear in mind that not all shifts in layout are bad or a cause for concern. Many modern websites are highly interactive and dynamic. If an element or content moves position as the result of a user hovering over an item, or clicking a button, this is ok. This is classed as an expected layout shift.
Google’s CLS scoring measure. Credit: Google

How can CLS affect SEO?

CLS or Cumulative Layout Shift, along with other Core Web Vitals metrics, have become an important addition to any SEO strategy.

User experience is a massive focal point for Google in developing their algorithms, and these metrics are designed to help website owners or agencies improve their sites to benefit users.

SEO in 2021 and beyond, therefore needs to focus around user experience,  on top of other on-page SEO factors.

Years ago, SEO was very much just adding keywords into pages to show relevance of a website. Optimisation is now so much more complicated and advancing rapidly year on year.

Putting yourself in the position of the user should be the priority in any SEO campaign, and has become a necessary revision in most agency SEO packages.

Making Cumulative Layout Shift Improvements

While meeting and surpassing the levels need to meet the Core Web Vitals standards can be daunting. Google have made it possible for most website owners with their measurement tools, mentioned above.

Even for someone who maintains a website themselves and has little SEO experience. Making small adjustments and rechecking how your site fares, is great place to start.

Begin with looking for obvious problems. Load your web pages and look for obvious delays in elements loading that may be causing other elements to shift position.

Often the majority of CLS problems are apparent by simply visiting each web page.

Take a look at why elements are behaving this way and the solution may be straightforward.

Example: Do you have a large, high quality image that takes a long time to load? Consider adding a placeholder for that image, effectively reserving its position and removing the content shift when it fully loads.

Other Core Web Vitals metrics

Cumulative Layout Shift is just one of the three main metrics, designed to help improve user experience on a web page.

It’s possibly the most confusing one, as  the other two can seem quite straightforward in comparison.

These are the other two Core Web Vitals metrics, with a brief expiation.

  • First Input Delay – Interactivity. Measures the time from when a user first interacts with a page, to the time when the browser is able to begin processing a response.
  • Largest Contentful Paint – Page Loading. Measures the loading performance of a page, focused on the time to render the largest image or content block visible on the page.


These two metrics, along with Cumulative Layout Shift with be the pillars of the Google update in May 2021.

Core Web Vitals Metrics. Credit: Google


Core Web Vitals, including the largely misunderstood Cumulative Layout Shift, are the new metrics to add to Google’s ranking algorithms of over 200 factors.

The metrics are designed to create a better experience for Google’s users.

Why send your valued users to a website that is slow to load, delayed interaction or elements moving around on the page?

That’s the goal, to help website owners make the most of their website, and keep their visitors, generating enquires and sales.

What do you think?

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