Google updates cumulative layout change score

Google fixed a Core Web Vitals metric to make it more accurate for certain types of sites. With about a month until Core Web Vitals becomes a ranking factor, some may find it strange that the calculation issues are still being updated.

Cumulative Layout Shift (CLS)

Have you ever visited a webpage and tried to click a button or read the text, only to have the button change places and the text scroll across the webpage?

This is called layout change, when the web page is unstable and moves.

Cumulative layout shift (CLS) is a measure of the amount of skips of webpage elements on a webpage.

The cause of the offset can be images whose height and width are not declared in the HTML. As the image uploads, the space around it expands, displacing text and other webpage elements.

A user cannot interact with a moving webpage. This metric is intended to measure the movement of a page as it downloads or as a visitor scrolls.

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

Fault in CLS calculations

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

Google noted:

“It’s important for the metric to focus on the user experience throughout the life of the page, as we’ve found that users often have negative experiences after loading, when scrolling or navigating through the pages.

But we’ve heard concerns about the impact of this on long-lived pages, pages that the user has typically been open for a long time.

Changing CLS will not worsen scores

Google looked at three solutions to update its CLS score. Each solution did not worsen the websites scores. So there is 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 to measure CLS

The measurement of page elements is measured in “session windows.” Session windows are the different parts of a web page that a user accesses when scrolling the web page.

The total scores for each session window are called the cumulative layout lag, the total lag of the entire page.

According to Google:

“A session window begins with the first layout change and continues to expand until there is a space with no layout change.

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

Many CLS scores will change

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

About 3% of web pages that use infinite scroll or have UI handlers that are slow to respond to user interaction will see their scores hit a “Well” Evaluation.

Update makes CLS scores more accurate

It’s a benefit for publishers that the new rating system is fair, especially for web pages that have been open for a long time (long running) or use infinite scroll and have been unfairly rated because of it.

Since the Core Web Vitals metrics will become a ranking factor in May 2021, this is also an important change to make almost at the last minute.


Official Google announcement:

Evolve the CLS metric