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

[Ryosuke Niwa]

On Wed, Feb 26, 2020 at 11:29 AM Maciej Stachowiak <mjs at apple.com> wrote:

>
>
> On Feb 26, 2020, at 10:51 AM, Noam Rosenthal <noam.j.rosenthal at gmail.com>
> wrote:
>
>
>
> On Wed, Feb 26, 2020 at 8:08 PM Maciej Stachowiak <mjs at apple.com> wrote:
>
>>
>> Some quick comments:
>>
>
>> the definition of First Contentful Paint here in the spec: <
>> https://www.w3.org/TR/paint-timing/#sec-terminology> does not match the
>> definition stated at <https://web.dev/first-contentful-paint/>. The
>> Chrome definition on web.dev specifies that iframe content is not
>> included, the spec does not have this limitation. Would an implementation
>> that matches the spec match Chrome?
>>
> The draft version of the spec specifies that iframe content is not
> included in FCP:
> https://w3c.github.io/paint-timing/#sec-reporting-paint-timing, and has a
> few more comprehensive details about this. I think it's a good place to
> start.
>
> I am also not sure this matches the layout milestones that already exist
>> in non-Blink browser engines. Has this spec been implemented in Gecko, for
>> example, to verity that it’s not exposing a concept that only exists in
>> Blink?
>>
> No, this has not been implemented in Gecko, I'm tracking the bug on this:
> https://bugzilla.mozilla.org/show_bug.cgi?id=1518999, there was some
> movement recently.
>
> I suggest to start from "first-paint", and to try to match chrome as much
> as possible in how FCP is implemented, in the cases where the spec doesn't
> give enough detail, if such places exist. I think that for the main
> use-case of catching regressions for website code, it's ok (and almost
> unpreventable) if the implementations have some variances between them,
> what matters is that the metric is reliable for the particular browser.
> I also suggest to start with "first-paint" as it's perhaps a bit less
> "internal" than FCP, and can provide a performance-regression metric with a
> lesser degree of risk regarding exposing internals / privacy.
>
>
> First paint that’s not first meaningful/contentful paint is not a very
> good performance metric IMO. Who cares that a paint happened if it doesn’t
> have any image or text content?
>
> I also don’t think this exposes less. The privacy risk here is exposing
> timing data that might be usable for fingerprinting.
>
>
>
>>
>> Chrome team themselves have been telling web developers that First
>> Contentful Paint is deprecated in favor of Largest Contentful Paint. Should
>> we concerned about this? It seems even harder to define LCP in an
>> engine-independent way.
>>
> What was deprecated was "first meaningful paint" (FMP). FCP was not
> deprecated and has been in wide use for some time.
>
>
> What’s the difference between First Meaningful and First Contentful?
>

There is no difference in Safari because we don't do any painting of a
newly navigated until the first meaningful paint happens.

- R. Niwa


>> Finally, we should do a privacy review to consider whether exposing this
>> info to webpages creates fingerprinting risk or otherwise exposes user data.
>>
> Great, what is needed for such review?
>
>
> We will discuss with Apple’s privacy experts what they think of the
> privacy risk. I’m just giving you a rain check for results of this
> discussion.
> _______________________________________________
> webkit-dev mailing list
> webkit-dev at lists.webkit.org
> https://lists.webkit.org/mailman/listinfo/webkit-dev
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.webkit.org/pipermail/webkit-dev/attachments/20200226/47d15760/attachment.htm>