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

Noam Rosenthal noam at webkit.org
Sun Mar 1 16:44:08 PST 2020

On Mon, Mar 2, 2020 at 2:37 AM Maciej Stachowiak <mjs at apple.com> wrote:

> On Mar 1, 2020, at 4:19 PM, Noam Rosenthal <noam at webkit.org> wrote:
> OK, to summarize what I got from this
> - we want the spec and webkit painting to be as close as possible
> - The spec needs to be clearer/less buggy about a few things, such as
> "White" canvas, spec issues to be files
> - WebKit should be closer to the spec wrt canvas, backgrounds and
> potentially pixel/character threshold (TBD)
> - FP is more sensitive than FCP, because it exposes browser differences
> and may lead to unwanted comparisons and misunderstanding. Thus, it should
> be exposed as a different runtime feature flag.
> One thing I'm wondering about - would it be better to change the rendering
> heuristics together with implementing the paint API reporting? Or would it
> be better to separate those concerns a bit in terms of implementation? I
> mean, having the performance APIs in the code behind 2 flags with failed
> tests conforming to the spec might help iterate on the actual rendering.
> What would you consider a better approach here?
> If I were doing this myself, I’d first change rendering heuristics
> (probably without a flag) and then add the web-facing API.
Okay, let's see how that goes.

> What you describe might be reasonable if there are WPT conformance tests
> that can distinguish cases where the FCP timing is too late or too soon.
> It’s probably possible to make such tests, but I am not sure if they
> already exist.
They exist to some extent, webkit currently fails 4 out of 13 painting-API
conformance tests, around some of the issues discussed here (e.g. one of
the tests fails because webkit doesn't consider background image to be
contentful). I think I'll work on a few more while working on the spec,
they're quite easy to write.

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