[css-contain] Container queries wrongly assume that all elements can be contained

[Loirooriol]

See https://drafts.csswg.org/css-contain-3/#container-type

• size
  Establishes a query container for container size queries on both the inline and block axis. Applies layout containment, style containment, and size containment to the principal box.
• inline-size
  Establishes a query container for container size queries on the container’s own inline axis. Applies layout containment, style containment, and inline-size containment to the principal box.

While all elements accept style containment, layout containment requires all these conditions:

• the element generates a principal box
• its principal box is not an internal table box or is a table-cell
• its principal box is not an internal ruby box nor a non-atomic inline-level box

Size containment and inline-size containment, in addition to the requirements above, also need:

• its inner display type is not table
• its principal box is not a table-cell

It makes no sense to establish a query container for container size queries if size containment has no effect on the element. Obvious circularity issues.

[andruud]

The intent was that an element with container-type:size/inline-size is always a query container, regardless of whether or not containment can apply on the element. However, any queries against a query container that "does not support" queries along the relevant axis results in "unknown". I prefer keeping it this way, since it makes it possible to answer the question "is this a query container" immediately after the style recalc of an element. If we involve layout box information, we can't answer that question until after layout-tree-building, which is annoying.

But "does not support" is a little vague, and we could minimally also link to https://drafts.csswg.org/css-contain-2/#size-containment-box explicitly.

[Loirooriol]

Ah, thanks, I missed

If [...] the query container does not support container size queries on the relevant axes, then the result of evaluating the size feature is unknown.

That works too. But yeah, the conditions for supporting container size queries are not clear, filed #7702