Cumulative Layout Shift (CLS)
Definition
Cumulative Layout Shift (CLS) measures how much visible content moves unexpectedly while a page loads or is used. It is a score with no unit, not a time, and Google advises keeping it at 0.1 or less.
A shift happens when something above or beside the content you are reading arrives late and pushes it. The usual causes are images, videos and embeds without set dimensions, banners and cookie notices inserted into the page flow, adverts and chat prompts that appear after load, and web fonts that swap in and reflow the text. The visitor feels it as tapping the wrong button just as the page jumps.
Reserve space for anything that arrives late. Give every image and video a width and height or an aspect ratio, put cookie and promo bars where they overlay the page without moving it, and give embeds a fixed container. Pass or fail is judged at the 75th percentile, with phones and desktops scored apart, and shifts that arrive in quick succession add up to one score.
Arabic pages deserve their own test. Right-to-left templates often use different fonts and line heights, and a late Arabic font can reflow every paragraph, so a page that passes in English can fail in Arabic. Check both language templates in Search Console, which groups similar pages together.
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