Definition
Cumulative Layout Shift is a Core Web Vitals metric measuring how much visible content unexpectedly moves around as a page loads and settles - text jumping down as an image or ad loads above it, a button shifting just as the user goes to tap it. It is scored from the size of the shifted area and the distance it moved, summed over the page's lifecycle (in the worst session window).
A good CLS is 0.1 or less; above 0.25 is poor.
CLS is caused by elements without reserved space - images and iframes without width and height attributes, ads and embeds that push content when they load, web fonts that reflow text when they swap in, and content injected above existing content after load.
In context
For an iGaming affiliate, CLS problems commonly come from the elements a monetised content site tends to have: display ad slots that push content when they fill, operator logos and screenshots without dimensions, embedded widgets and comparison tables that resize on load, cookie and age-gate banners that appear and shift the page, and web fonts. A page that jumps around while loading is annoying on any device and especially on mobile, and it directly hurts the page-experience signal.
The fixes are mostly about reserving space: set explicit width and height (or CSS aspect-ratio) on all images, iframes and media; reserve fixed space for ad slots, embeds and widgets so they fill without pushing content; load the consent and age-gate UI in a way that does not reflow the content (overlay rather than insert, or reserve space); use font-display and preload key fonts to minimise text reflow; and never inject content above existing content after load. CLS should be checked as field data, since it depends on real ad fill, real fonts loading and real interaction timing.
For affiliate-facing content, the framing is that CLS measures unexpected movement of visible content during load, that a good score is 0.1 or less, that on affiliate sites it usually comes from unsized images, ad slots, embeds and banners, and that the fix is reserving space for every element that loads or changes size.
Worked example
An affiliate's field data shows a CLS of 0.28 on review pages, mostly from ad slots and unsized operator logos pushing content, plus a cookie banner inserting above the article. The team sets dimensions on all images, reserves fixed height for ad slots and embeds, and switches the cookie banner to an overlay; CLS drops to 0.05.
Related terms
Frequently asked questions
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