Performance
Cumulative Layout Shift (CLS)
A measure of how much a page's content unexpectedly moves around while it loads.
CLS quantifies visual instability - the annoying jumps where you go to tap a button and it shifts because an image or ad loaded above it. Google considers a score of 0.1 or less to be good.
It's usually caused by images and embeds without dimensions, late-loading fonts, or content injected above existing content. Reserving space for elements before they load is the main fix.
Further reading
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