[webkit-dev] WebKit position on Paint Timing / (first paint, first contentful paint)

[Noam Rosenthal]

On Tue, Mar 3, 2020 at 10:18 AM Ryosuke Niwa <rniwa at webkit.org> wrote:

> Sorry for the delay. I had other other things to take care of first.
>
> Based on the discussion we had (between Maciej, Simon, Alan, and I), we
> should take the following items into account for WebKit's first meaningful
> paint heuristics:
>
>    - Background image
>
> I've filed https://bugs.webkit.org/show_bug.cgi?id=208501 and can get it
done.
Btw if there's something I'm taking on myself but Apple would rather do or
vice versa, please let me know :)

>
>    - SVG images
>    - "Contentful" canvas once we rigorously defined what it means:
>    https://github.com/w3c/paint-timing/issues/50
>    - First frame / poster image of a video:
>    https://github.com/w3c/paint-timing/issues/51
>
> Then as Maciej pointed out there are a few spec works to do:
>
>    - WebKit takes any text regardless of whether they appear within UA
>    shadow root or not into account for the first meaningful paint. The spec
>    needs to clarify this behavior -
>    https://github.com/w3c/paint-timing/issues/52
>    - The exact timing of navigation should be defined -
>    https://github.com/w3c/paint-timing/issues/19
>    - Clarify whether "first paint" or "first content paint" ever happens
>    to a blank page - https://github.com/w3c/paint-timing/issues/53
>    - Clarify what happens to a page which consists of just an iframe -
>    https://github.com/w3c/paint-timing/issues/54
>    - Combination of paint timing and long tasks can expose precise paint
>    timing - https://github.com/w3c/paint-timing/issues/55
>
> To supplement earlier Maciej's points, per our discussion, we don't think
> "first paint" is a good metric to expose to the Web since Safari / WebKit
> users would never see that. If any website optimize for the first paint
> metrics instead of or at the cost of first contentful paint, then such an
> optimization would only result in perceived regressions for our users.
>
I've spoken with the Wikipedia folks on this and they agree, first-paint is
not really that useful as a performance metric (I think it's useful to
catch bugs, e.g. in cases where it's vastly behind first-contentful-paint).
For now I'm focusing only on the first-contentful-paint metric, and adding
web platform tests to cover this situation (the current tests would fail in
the case where FP is not implemented).
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