Cumulative Layout Shift Sources are the specific DOM elements responsible for unexpected visual movement that contributes to a page’s CLS score, which is an official Core Web Vital. The Layout Instability API, available in Chromium-based browsers, exposes each layout shift entry with a list of contributing sources including the element that moved, its previous position, and its current position. Common CLS sources include images and media without explicit width and height attributes, dynamically injected content such as ad slots or cookie banners, web fonts that cause FOUT or FOIT during load, and CSS animations that trigger layout rather than composite-only operations. Google’s tools — including Chrome DevTools’ Performance panel, Lighthouse, and the Web Vitals extension — surface CLS sources so developers can trace the exact elements causing shifts. Fixing CLS typically involves reserving space for dynamic content, setting explicit dimensions on media, and using CSS transform instead of properties that trigger layout recalculation.