Re: Specifying the calculation of INP from Event Timing entries

It's an interesting question.  The web platform specifications define the
public web platform features: Event Timings (for INP), Layout Instability
(for CLS).  Largest Contentful Paint the web feature is more closely
matching LCP the metric (though even there, there are gaps, like merging
iframes).

I think there is not typically a "spec" for INP/CLS other than the specific
conventions that the Core Web Vitals program uses, and those are defined on
sites like web.dev (as you linked) and reference implemented on
web-vitals.js.  Other RUM providers I think have always deviated in small
ways (i.e. loading-only vs post-load, etc) and might always need to deviate.

That said: we do have a note in the non-normative section
<https://wicg.github.io/layout-instability/#cumulative-layout-shift> of
layout instability spec for DCLS and CLS, with a usage example -- though
that description isn't up to date with the latest CLS CWV metric...

I would be happy if someone wanted to add documentation to all the
non-normative sections of all the specs to define these, but I'm not also
not sure about it.

On Tue, Feb 4, 2025 at 12:13 PM Andy Davies <andy.davies@speedcurve.com>
wrote:

> https://web.dev/articles/inp contains an abstract overview of what INP
> represents but…
>
> There's no actual specification for how INP it should be calculated from
> Event Timing entries.
>
> Don't know whether this should exist within this CG or be raised in the
> WebPerf WG but as it's a 'standard' metric then I think it should be
> documented
>
> Thanks
>
> Andy
>
> --
>
> Andy Davies
> Web Performance Consultant, SpeedCurve
>
>
>