v133 · performance

Resource timing: revert responseStart change and introduce firstResponseHeadersStart

Resource timing: - responseStart returns the first response, either early hints (interim) or final - Expose the final response headers (2xx/4xx/5xx) time as finalResponseHeadersStart.

concepts

  1. firstResponseHeadersStart

    Resource Timing reverts the responseStart definition tweak and adds firstResponseHeadersStart for the new use case. Restores cross-browser parity for existing RUM.

  2. Early hints 103 timeline

    A live PerformanceNavigationTiming timeline showing requestStart, responseStart, and the new finalResponseHeadersStart on the same axis — with table of the field semantics across the three eras.

  3. Timeline decoder

    Pick a response scenario; read what each field returns under the three eras (pre-130, reverted attempt, 133 final) and watch the timeline bar scale.

  4. RUM migration guide

    Exact code diff for updating a Real User Monitoring TTFB calculation to use finalResponseHeadersStart ?? responseStart, with a live PerformanceObserver showing real field values from your browser.

why it shipped

With early hints (103), responses have two timestamps: - When the early hints arrive (103) - When the final headers arrive (e.g. 200)

references