How INP and it's sub-parts are calculated from Event Timing entries is not specified / documented

[andydavies]

INP and it's sub-parts are calculated from Event Timing entries but there is no specification for the process used to calculate them.

This lack of documentation can lead to differences between implementations – as implementations are all starting with the same stream of events they should all generate the same output

It also leads to a lack to traceability when the process changes, for example, Chrome changed their approach to consider all Event Timing entries within the same frame when calculating the INP sub-parts but didn't document this