"Google originally announced the Core Web Vitals in 2020. It's a set of three metrics that measure website speed and user experience. Today, these metrics are: Largest Contentful Paint (LCP): how fast does the page load? Interaction to Next Paint (INP): how quickly does the page respond to user input? Cumulative Layout Shift (CLS): is page content stable after rendering?"
"However, the Core Web Vitals were still mostly Chrome-only metrics. Firefox and Safari started work on the Largest Contentful Paint and Interaction to Next Paint metrics. Support for Cumulative Layout Shift is not currently planned, although there is a proposal to include it in Interop 2026. Firefox has supported Interaction to Next Paint since the release of version 144 in October 2025. Safari has started to implement the two metrics, although they are currently only available in the Safari Technology Preview version ( INP, LCP)."
Core Web Vitals, introduced in 2020, measure page load speed, input responsiveness, and visual stability via LCP, INP, and CLS. LCP evaluates how fast main content renders, INP gauges responsiveness to user input, and CLS assesses layout stability after rendering. Metrics were primarily Chrome-centric, with Firefox adding LCP support in January 2024. Interop 2025 prompted cross-browser implementation of LCP and INP; Firefox shipped INP support in version 144 (October 2025) and Safari began implementations in Safari Technology Preview. CLS support is not in Interop 2025, with a proposal aiming for Interop 2026. Lighthouse and Chrome DevTools supply measurement and recommendations.
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