[css-conditional-5] Clarify behavior of named container queries in shadow trees

[frehner]

The spec currently says

Style rules applying to a query container’s flat tree descendants can be conditioned by querying against it, using the @container conditional group rule.

Which was aligned here #5984 (comment)

The spec also does not carve out an exception for named container queries, so my assumption would be that they should also follow the same rule.

However, WPT has tests specifying that named container queries set inside of a shadow tree do not behave this same way, and currently all browsers are (mostly) compliant with those tests.

If this carve-out was intentional for named container queries, I think it would be nice for the spec to be updated to reflect that. If it's not intentional, then perhaps the WPTs should be reversed.

[mirisuzanne]

For context: The initial spec made containers inside shadow DOM entirely hidden from light-style queries. The theory was that internal containers are an implementation detail, and shouldn't be exposed. That makes some sense, except that there's no way to select the slot of an element, and query that as a container. So this often required extra elements (similar to grid cells, I suppose).

We reverted that because it seemed like container units should resolve on the flat tree, and that container units should resolve the same as queries. I also argued that containers are about getting information from context – much like inheritance, which works on the flat tree. Style queries make that association even more explicit, since the goal is often to query the value that will be inherited on a property.

In making that change, we didn't specify the question of container names. It does seem to me though that "Container queries and units use the flat tree" would include container names. That seems to me like the WPT is out of date.

[frehner]

I believe @lilles added the container name WPT tests here web-platform-tests/wpt#46687 which looks like it originated in Chromium.

Maybe there's some context we can gain there?