Expose PerformanceResourceTiming.firstInterimResponseStart In cases where a navigation or subresource fetch encounters an interim 1xx response, e.g. 100 continue or 103 early hints, firstInterimResponseStart would now correspond to the time of that first interim response, while responseStart would correspond to the final response, e.g. with 200 status.
Since the introduction of 103 Early-Hints, the meaning of responseStart (aka TTFB) in resource timing has changed for pages that support it, to mean the "first 103 response". This has caused confusion, and web developers that rely on TTFB gave feedback that the new meaning is unexpected. To mitigate, introducing 3 new values to resource timing: - firstInterimResponseStart, the time of the first 103 response if exists - responseHeadersEnd, when the headers are done - responseBodyStart, when the body starts streaming. responseStart/TTFB would mean the finalResponseStart, as it was before 103 early hints.