[css-contain-3] Define a syntax for state-based container queries #6402

In the discussion of what container features can be queried, there was interest in queries based on the "current state" of a container – such as whether a position: sticky container is currently "stuck" (displaced from its in-flow position), or whether the box is currently visible on screen.

In the draft of CSS Containment L3 we currently allow a state value for container-type, but different states may require different types of containment (or none at all), so I don't think that single type value will work. We also need to define which states are supported initially, and how these states are queried in the @container rule.

For example, something along the lines of:

header {
  container: is-stuck is-visible / header;
  position: sticky;
  top: 0;
}

@container header (is-stuck) { /* … */ }
@container header (is-visible) { /* … */ }