Google Updates Cumulative Layout Shift Scoring

Google fixed a Core Web Vitals metric in order to make it more fair to certain kinds of sites. With about one month to go before Core Web Vitals becomes a ranking factor some may find it strange that issues in the calculations are still being updated.

Cumulative Layout Shift (CLS)

Ever visit a web page and try to click a button or read the text, only to have the button shift places and the text march around the web page?

That’s called layout shift, when the web page is unstable and shifting around.

Cumulative Layout Shift (CLS) is a measurement of how much the web page elements jump around on a web page.

The cause of the shifting can be images that don’t have height and width declared in the HTML. When the image downloads it causes the space around it to expand, moving the text and other web page elements around.

A user can’t interact with a web page that is moving around. This metric is meant to measure how much a page moves around while it’s downloading or a visitor is scrolling.

The ideal result is to have a page that downloads and is stable from the moment the site visitor views it.

Flaw in CLS Calculations

Google received feedback that the CLS metric was inadequate for measuring web pages that are open for a long time, penalizing those pages with lower scores.

Google noted:

“It’s important that the metric focuses on user experience through the full page lifetime, as we’ve found that users often have negative experiences after load, while scrolling or navigating through pages.

But we’ve heard concerns about how this impacts long-lived pages— pages which the user generally has open for a long time.”

Change in CLS Will Not Make Scores Worse

Google reviewed three solutions for updating how it scored CLS. Each solution did not make scores worse for websites. So there’s no need to worry that CLS scores will get worse as a result of this change.

Session Windows for Measuring CLS

Google chose a Session Windows approach for measuring CLS

The measurement of the page elements is measured in “session windows.” The session windows correspond to different parts of a web page that a user reaches as they scroll down the web page.

The total scores for each session window is called the cumulative layout shift, the total shifting of the entire page.

According to Google:

“A session window starts with the first layout shift and continues to expand until there is a gap with no layout shifts.

When the next layout shift occurs, a new session window starts. …Similar to the current definition of CLS, the scores of each shift are added up, so that each window’s score is the sum of its individual layout shifts.”

Many CLS Scores Will Change

According to Google, most web pages (55%) will not see a change in their cumulative layout shift scores. About 42% of sites will see a small improvement in scores.

About 3% of web pages that use infinite scroll or have user interface handlers that are slow to react to user interaction will see their scores rise to a “good” rating.

Update Makes CLS Scores More Accurate

It’s a plus for publishers that the new scoring system is fair, particularly to web pages that are open for a long time (long lived) or employ infinite scroll and were being unfairly scored because of that.

Given that Core Web Vitals metrics will become a ranking factor in May 2021 it’s also somewhat of an important change to make at virtually the last minute.


Official Google Announcement:

Evolving the CLS Metric