Resource timing: - responseStart returns the first response, either early hints (interim) or final - Expose the final response headers (2xx/4xx/5xx) time as finalResponseHeadersStart.
With early hints (103), responses have two timestamps: - When the early hints arrive (103) - When the final headers arrive (e.g. 200) When we shipped firstInterimResponseStart (https://chromestatus.com/feature/5086730938482688), we also changed the meaning of responseStart to mean "the final headers". This created confusion because other browsers didn't do the same, and also CrUX wasn't change in the same way. As resolved in the WebPerfWG, changing this to: - revert the meaning of responseStart to mean the first post-redirect response, be it 1xx/2xx/4xx/5xx. - Adding finalResponseHeadersStart that specifically means the 2xx/4xx/5xx response. - Keeping firstInterimResponseStart, which returns the 1xx response time or 0 if that didn't exist. This aligns back with the other browsers, while adding a useful piece of information (the final response time).