> On Feb 26, 2020, at 10:51 AM, Noam Rosenthal <noam.j.rosent...@gmail.com> 
> wrote:
> 
> 
> 
> On Wed, Feb 26, 2020 at 8:08 PM Maciej Stachowiak <m...@apple.com 
> <mailto:m...@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 
> <https://www.w3.org/TR/paint-timing/#sec-terminology>> does not match the 
> definition stated at <https://web.dev/first-contentful-paint/ 
> <https://web.dev/first-contentful-paint/>>. The Chrome definition on web.dev 
> <http://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 
> <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 
> <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?

>  
> 
> 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.
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