Core Web VitalsSimple mode
CLS
Measures Cumulative Layout Shift, the score that captures unexpected visual movement while the page loads.
Why it matters
A page that jumps around feels broken even when it loads quickly. CLS is a Core Web Vital and is often harmed by images without dimensions, injected UI, unstable embeds, or late-loading fonts.
Where to run it
Available automatically in Simple mode.
Can also be explored in more depth from the underlying WebPerf Snippets reference.
Open analyzerWhat you learn from this test
Whether visible elements are moving enough to disrupt reading or clicking
Whether your page stays visually stable during load
Whether layout shifts are likely hurting trust and conversion
Common fixes
Set width and height on images, videos, and embeds
Reserve space for banners, widgets, and ads before they load
Improve font loading to reduce text reflow
Avoid injecting content above existing content after render