CLS.

CLS stands for Cumulative Layout Shift. It measures the visual stability of a webpage by tracking how much visible content unexpectedly changes position while someone is using the page. A layout shift can happen when an image loads without reserved space, a web font changes the size of text, an advertisement appears above existing content, or a late-loading section pushes buttons and paragraphs down the page. For example, a visitor may try to tap a checkout button just as a banner loads above it. The button moves, and the visitor accidentally taps something else. CLS is designed to measure this kind of unexpected movement. CLS is a unitless score rather than a measurement in seconds. A score of 0.1 or less is considered good, a score above 0.1 and up to 0.25 needs improvement, and a score above 0.25 is considered poor. Not every movement counts as a bad layout shift. Content that moves immediately after a user deliberately opens a menu, expands a section or performs another interaction may be expected and may not be included in the score. Common ways to improve CLS include reserving width and height for images and videos, leaving space for advertisements and embeds, avoiding content that suddenly appears above existing content, and loading fonts carefully.

Poor CLS makes a website feel unstable and difficult to use. Visitors may lose their reading position, click the wrong button or struggle to complete a form because page elements keep moving. CLS is one of Google’s Core Web Vitals and is used as part of its broader page-experience ranking systems. However, CLS is not a standalone penalty, and achieving a good score does not guarantee that a page will rank highly. The main reason to improve CLS is to give users a stable and predictable experience. It should be reviewed alongside other performance measures such as Largest Contentful Paint and Interaction to Next Paint.