Intent to Ship: CSS sibling-index() and sibling-count()

I'm very slightly worried about the cases which we seem to accept, but the latest on the CSSWG thread suggests we should disallow. Namely, @container and @page. How sure are you that changing those to be invalid in the future, to follow the latest CSSWG decisions, will not cause compat problems?

On Tuesday, May 13, 2025 at 6:37:14 AM UTC+9 Mike Taylor wrote:

LGTM1

On 5/9/25 9:47 AM, Rune Lillesveen wrote:

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On Tue, May 13, 2025 at 7:43 AM Domenic Denicola <dom...@chromium.org> wrote:

I'm very slightly worried about the cases which we seem to accept, but the latest on the CSSWG thread suggests we should disallow. Namely, @container and @page. How sure are you that changing those to be invalid in the future, to follow the latest CSSWG decisions, will not cause compat problems?

For @page, I wouldn't be worried at all. It's unlikely someone will start using the feature and rely on a constant sibling-index()/sibling-count() in @page.

For @container, I agree that it's safer to be conservative and wait for the resolution, since for @container there are clear use cases and and a more or less obvious behavior in that context.

On Tue, May 13, 2025 at 8:34 AM Rune Lillesveen <fut...@chromium.org> wrote:

On Tue, May 13, 2025 at 7:43 AM Domenic Denicola <dom...@chromium.org> wrote:

I'm very slightly worried about the cases which we seem to accept, but the latest on the CSSWG thread suggests we should disallow. Namely, @container and @page. How sure are you that changing those to be invalid in the future, to follow the latest CSSWG decisions, will not cause compat problems?

For @page, I wouldn't be worried at all. It's unlikely someone will start using the feature and rely on a constant sibling-index()/sibling-count() in @page.

For @container, I agree that it's safer to be conservative and wait for the resolution, since for @container there are clear use cases and and a more or less obvious behavior in that context.

Some more details below.

For @container, this is relevant for size queries and style() queries.

Size queries are currently always evaluated in an element context, although falling back to viewport has been discussed, and container units fall back to small viewport units. Relative units (like ems below) are evaluated against the computed values of the container element:

@container (width > calc(sibling-index() * 50px)) {}

@container (width > 10em)) {}

For style queries, the right hand of the query is evaluated against the container element and its computed styles for registered custom properties. Note that for non-registered custom properties, sibling-index() would just be part of the string/tokens without any specific meaning.

I think it would be inconsistent to reference relative units (like em below) and resolve custom properties references (like var(--a) below), but specifically throw away sibling-index() when evaluating the value against the registered syntax:

@container style(--registered-length: calc(sibling-index() * 20px)) {}

@container style(--registered-length: 10em) {}

@container style(--registered-length: var(--a)) {}

and if we're confident the right solution is to match how relative units, then we can proceed. Rune pointed out that it's already tested here:

https://wpt.fyi/results/css/css-values/tree-counting/sibling-function-container-query.html

To ship without this behavior only to add it a few milestones later would complicate the browser support story and require explanation on places like MDN and caniuse.com.

In case the CSSWG decision is made before 138 ships to stable and it does not align with what you're proposing we ship, are you OK with disabling the feature using its Finch flag? Or should we put @container support behind a separate flag?