v132 · 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

    Adds the new field, reverts the prior responseStart tweak.

  2. early hints

    The motivating 103-then-200 scenario as a tunable timeline; the two fields cleanly separate.

  3. live timing anatomy

    Fetch any URL; the page reads its PerformanceResourceTiming entry and surfaces all 12 milestones (including the new v132 firstResponseHeadersStart) on a live axis.

  4. Timing Waterfall

    Four scenarios — simple HTTP, 103 Early Hints, 100 Continue, slow TTFB — rendered as a Resource Timing waterfall. See exactly where firstInterimResponseStart and the reverted responseStart land on each timeline.

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