Bug 304555 – Named Style Container Queries on :host fail for Slotted Elements

• Status: NEW
• Component: CSS

Safari 26.2 now resolves container-name matching regardless of origin, a discrepancy remains: named style container queries defined on :host are not applied to slotted elements.

While unnamed queries were recently fixed for the composed tree, https://github.com/WebKit/WebKit/commit/a20439aa1afd56e3b8e0f4af7c899ac98237fa3d, the same logic fails in Safari when a container-name is involved. 

Currently, ✅Chrome correctly evaluates the query,  but ❌Safari (26.2 and Safari Technology Preview 234) requires the container to be defined in the Light DOM to work.


Reproduction:

 https://lit.dev/playground/#gist=8601556f4a13e49eb85743791f21157d

Steps to reproduce:

1. Open the playground in **Safari** and **Chrome**.
2. Observe that in Safari, the background color is **not** applied to the slotted element.
3. In `index.html`, uncomment the following block:

```css
parent-element {
  /* container-name: slotted; */
}

```

4. Notice it now works in Safari. This confirms the issue only exists when the container is defined via `:host` in the Shadow DOM.

- Expected Behavior: Slotted elements should resolve container queries against a named container defined on the `:host`

- Actual Behavior (Safari): Slotted elements ignore named containers defined on `:host`



References

- Related Fix: https://github.com/WebKit/WebKit/commit/a20439aa1afd56e3b8e0f4af7c899ac98237fa3d

- Relevant Commit: https://github.com/WebKit/WebKit/commit/efd4ebe98cab2bbb385ffe1e1e40d3870917727b

- Container-names are tree-scoped: https://bugs.webkit.org/show_bug.cgi?id=289868

[Anthony Frehner]

Was able to get this down to a bare reproduction; I have a PR up to WPT to add this test case here https://github.com/web-platform-tests/wpt/pull/56899

[Radar WebKit Bug Importer]

<rdar://problem/167244007>

[Anthony Frehner]

Also I plan on fixing this once the WPT addition is merged.