On Tue, Mar 3, 2020 at 10:18 AM Ryosuke Niwa <rn...@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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