[webkit-dev] content-visibility (Was Re: Request For Position on CSS containment)

cathiechen cathiechen at igalia.com
Wed Dec 22 01:23:06 PST 2021

Hi Ryosuke,

> On 25 Jun 2021, at 4:23 AM, Ryosuke Niwa <rniwa at webkit.org> wrote:
> Please rename the subject when you're going to discuss the work on a new feature.
> On Thu, Jun 24, 2021 at 9:44 AM cathiechen via webkit-dev <webkit-dev at lists.webkit.org> wrote:
> We made a lot of progress regarding CSS containment [1].
> Rob and I have finished the layout containment and size containment [2].\o/
> And the patches of paint containment and style containment are ready for review now [3].
> So we think now it's time to move on to content-visibility:
> (https://www.w3.org/TR/css-contain-2/#content-visibility)
> That seems premature. Have we implemented all the perf optimizations for layout, size, & paint containment? I'd rather not start piling on more features before we get to a point where we're happy with the performance of these features.

Thanks for the advice! Yeah, we kept working on the CSS containment performance, here is the summary (also sent to the previous thread)
https://docs.google.com/document/d/166CsE0BqOGur1UyeMqxC7OSijeH90R1Pdck54rLgZNU/edit?usp=sharing <https://docs.google.com/document/d/166CsE0BqOGur1UyeMqxC7OSijeH90R1Pdck54rLgZNU/edit?usp=sharing>

> Since content-visibility depends on paint and style containment, we will do some specification research first, then prototype it based on Rob's patches.
> Does the research part also include making a judgement call as to whether it's a good idea at all? It's wholly unclear to me that content-visibility is a feature we'd like to implement in WebKit given its implications to the accessibility and other browser features.

Yeah, while working on the performance, we found CSS Contain improves performance inside the containment box, for instance, setting relayout boundary to stop dirty layout propagating to ancestor elements. And we think content-visibility would improve the performance from outside, for instance, skipping painting content that is offscreen. And we asked for help from chromium team, they provided us some very positive feedbacks from the developers.(Also list in the document)

Regarding the accessibility feature, the values of content-visibility can decide whether or not the content is accessible.
And there are over 80 cases in WPT to test the behaviour of content-visibility.
Also we would like to analyse the performance in the future.

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