Cumulative Layout Shift
A Core Web Vital scoring unexpected visual movement of page content, computed from the largest burst of layout shifts; good is 0.1 or less…
TechnicalFOITFOUTfont-display
Web font loading
The strategy for downloading and applying custom fonts; poor handling causes invisible text (FOIT), layout shift (FOUT) and slow LCP for text-led pages.
In full
`font-display: swap` shows fallback text immediately and swaps when the font arrives, protecting FCP at the cost of a shift; `optional` avoids the shift by skipping late fonts. Self-hosting with `preload` and `crossorigin`, subsetting to the characters actually used, and matching fallback metrics with `size-adjust` and `ascent-override` reduce both the delay and the shift. Third-party font hosts add an extra connection and are no longer shared-cached across sites.
Example
Self-hosting a subsetted WOFF2 file and declaring `font-display: swap` with metric-matched fallbacks removes a 0.12 CLS contribution.
Related terms
First Contentful Paint
The time from navigation start until the browser renders the first piece of DOM content; a diagnostic metric rather than a Core Web Vital.
Preload
A resource hint that tells the browser to fetch a critical resource for the current page at high priority, before the parser would…
Largest Contentful Paint
A Core Web Vital measuring the render time of the largest image, video poster or text block visible in the viewport; good is 2.5 s or less…
Render-blocking resources
Stylesheets and synchronous scripts in the head that must be downloaded and processed before the browser can paint anything.