style: Improve CSS OM for CSSContainerRule

This implements https://github.com/w3c/csswg-drafts/pull/7293, pending a
spec resolution, so landing test as tentative for now, but the change
makes sense.

Differential Revision: https://phabricator.services.mozilla.com/D155602

components/style/stylesheets/container_rule.rs

@@ -47,6 +47,11 @@ impl ContainerRule {
         &self.condition.condition
     }
 
+    /// Returns the query name filter.
+    pub fn container_name(&self) -> &ContainerName {
+        &self.condition.name
+    }
+
     /// Measure heap usage.
     #[cfg(feature = "gecko")]
     pub fn size_of(&self, guard: &SharedRwLockReadGuard, ops: &mut MallocSizeOfOps) -> usize {
@@ -88,10 +93,12 @@ impl ToCssWithGuard for ContainerRule {
 }
 
 /// A container condition and filter, combined.
-#[derive(Debug, ToShmem)]
+#[derive(Debug, ToShmem, ToCss)]
 pub struct ContainerCondition {
+    #[css(skip_if = "ContainerName::is_none")]
     name: ContainerName,
     condition: QueryCondition,
+    #[css(skip)]
     flags: FeatureFlags,
 }