Cumulative Layout Shift (CLS)
Cumulative Layout Shift measures how much the content of a page moves around unexpectedly. You start reading, an image loads above the text and pushes it down; you go to tap a button, and a banner slides in so you hit something else. CLS puts a number on that.
CLS is one of the three Core Web Vitals. It has no unit: 0 means nothing moved, and the more and the farther content moves, the higher the value.
Why it matters
Section titled “Why it matters”Moving content is more than an annoyance. Visitors lose their place while reading, and they click the wrong link or the wrong button, sometimes one they didn’t want at all, such as an ad or “Buy”.
Not every movement counts. When content changes right after a visitor clicks or taps, for example when a menu opens, it’s expected and left out. CLS also doesn’t add up every shift over a long visit. It groups shifts that happen close together and reports the largest group.
What influences it
Section titled “What influences it”Browser: The most common cause is content that loads without reserved space: images, ads, embedded videos and iframes without a fixed width and height. The browser lays out the text first, and when the image or embed arrives, everything below it moves. Web fonts can cause smaller shifts when the text switches from the fallback font to the web font and takes up a different amount of space.
WordPress: WordPress adds width and height to images in posts and pages, but images in theme headers, page builder sections and sliders don’t always have them. Cookie notices, promotion bars and newsletter boxes that push the page down, rather than overlaying it, cause shifts. So do ads and embeds that load late, and sliders or carousels that rearrange themselves once their script has run.
The lab test only sees shifts during the load and in the first screen. Real visitors scroll, so images further down the page, loaded as they come into view, often cause shifts that only show up in field data.
In Permo
Section titled “In Permo”Permo shows CLS on its PageSpeed page, marked as a Core Web Vital, in the lab values for Mobile and Desktop and, when Google has data, in the Real users section: the value that 75% of Chrome visits reached or beat over the last 28 days.