Re: [csswg-drafts] [css-position-3] List all specified properties that establish a containing block for fixed-position elements (#12083)

Looking at the code references you provided, it seems that all browsers agree - or at least should agree - on a relatively fixed set of properties: `transform`, `will-change`, `contain`, `filter`, and `backdrop-filter`.

In particular, looking at Chrome’s codebase, I don’t see any substantial changes to this logic over the past six years (see [this 2019 reference](https://chromium.googlesource.com/chromium/src/+/4be7920fee20fd21f5c4dc432dc18d2579a487fd/third_party/blink/renderer/core/layout/layout_object.cc)). So, I don't find the "maintenance concern" particularly compelling. There’s no evidence that the list is changing frequently or at risk of becoming obsolete quickly.

I also didn’t quite follow your point regarding REC vs WD. Specifically, I don’t see why it would be "problematic" to update the list in the future if a new property is added. Specs are updated all the time - updating a list like this to reflect changes doesn't seem any more burdensome than updating other parts of a spec.

In any case, I suggest explicitly adding `filter` and `backdrop-filter` to the list. I also recommend removing the ellipsis at the end - it doesn’t really serve a purpose here. Ellipses are typically used to suggest a continuation based on a clear pattern, but there’s no such pattern here, so it just reads oddly.

If maintaining an exhaustive list is indeed a concern, I would suggest rephrasing the note as follows:

> Properties that can cause a box to establish a fixed positioning containing block include, but are not limited to, `transform`, `will-change`, `filter`, and `backdrop-filter`. This is not an exhaustive list; CSS properties that cause a box to establish such a containing block should document this behavior explicitly in their respective specifications.

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Received on Wednesday, 16 April 2025 14:25:23 UTC