Two Core Web Vitals reach cross-browser support

With Safari 26.2 shipping on December 12, 2025, Largest Contentful Paint (LCP) and Interaction to Next Paint (INP) are now Baseline Newly available. Every major browser engine now implements the Largest Contentful Paint API and the Event Timing API, closing a long-standing gap in how web performance can be measured. Both APIs were part of the Interop 2025 project.

Filling the measurement blind spot

The Core Web Vitals have become a common language for discussing page experience, both for developers and business stakeholders. They condense the complicated reality of web performance into three numbers: how fast a page loads (LCP), how quickly it responds to interaction (INP), and how visually stable it is (CLS).

Until now, those metrics could only be measured in Chromium-based browsers. That left a significant blind spot: on iOS, all browsers run on WebKit, so no data was available for any iPhone or iPad user. Many performance optimizations do benefit all engines, but rendering pipelines and interaction handling differ between browsers. Relying on a single engine's view of performance meant an incomplete picture of how a site actually behaves for its visitors.

With LCP and INP now measurable in all major browsers, site owners can finally compare loading and interactivity across the full user base. This makes it easier to spot problems and prioritize fixes, with the potential to improve both user experience and business outcomes.

What this doesn't change: CrUX data

The Chrome User Experience Report (CrUX) remains based solely on eligible Chrome users. That policy is unchanged, and it applies to everything built on CrUX data, including PageSpeed Insights, Google Search Console, and CrUX Vis. Chrome iOS continues to be excluded from CrUX, since those users run WebKit under the hood.

Measuring with your own RUM

CrUX remains a good high-level summary for benchmarking against the web at large. But for diagnosing and fixing performance issues, more detailed field data has long been recommended.

Real User Monitoring (RUM) tools can now capture this data from all major browsers. For most setups that use the web-vitals library, coverage should expand automatically once users upgrade their browsers. Check with your RUM provider if you're unsure about its cross-browser support. It's also worth remembering that RUM and CrUX can differ, and those differences may become more pronounced now that these metrics cover browsers outside the CrUX population.

Small implementation differences to know about

All browser engines do broadly the same job when loading and rendering pages, but their rendering pipelines differ under the hood. The end of the rendering loop is defined interoperably through the paintTime. However, Chrome also measures up to presentationTime, a later, implementation-specific timestamp that indicates when pixels actually appear on screen. Firefox and Safari stop at paintTime instead.

The result is a small timing difference, usually just milliseconds, between Chrome's measurements and those from Firefox or Safari. The same distinction applies to INP. Chrome 145 will expose paintTime for LCP as well, so comparisons across browsers can be made on the same basis.

Implementing these APIs in more engines has also surfaced edge cases that need clarification in the specifications. That work is tracked and may cause minor discrepancies, mostly in uncommon scenarios. The project continues under the Interop umbrella.

These differences are small enough that LCP and INP are considered broadly interoperable and deserve the Baseline Newly available label. Developers building on web-vitals should not notice meaningful deviations across browsers.

Dealing with older browsers

Baseline Newly available means support exists only in the latest versions of each major browser. Users who haven't upgraded, or can't due to OS or provider constraints, will lag behind. After 30 months, these metrics reach Baseline Widely available status as most users will be running supported versions.

Since these are measurement APIs and not core functionality, you can safely collect data from browsers that support them and ignore the rest. Just be aware that in the short term, your field data may overrepresent users on recent browser versions.

What about CLS?

Cumulative Layout Shift (CLS), the third Core Web Vital, is not part of Interop 2025. There is an Interop 2026 proposal, but it remains unsupported outside Chromium-based browsers for now.