style: Add a couple FIXMEs I've noticed while working on this.

::slotted is hard.

components/style/data.rs

@@ -262,6 +262,8 @@ impl ElementData {
         }
 
         let mut xbl_stylists = SmallVec::<[_; 3]>::new();
+        // FIXME(emilio): This is wrong, needs to account for ::slotted rules
+        // that may apply to elements down the tree.
         let cut_off_inheritance =
             element.each_applicable_non_document_style_rule_data(|data, quirks_mode| {
                 xbl_stylists.push((data, quirks_mode))

components/style/invalidation/element/invalidator.rs

@@ -505,6 +505,9 @@ where
                 DescendantInvalidationKind::Slotted,
             );
 
+            // FIXME(emilio): Need to handle nested slotted nodes if `element`
+            // is itself a <slot>.
+
             debug_assert!(
                 sibling_invalidations.is_empty(),
                 "::slotted() shouldn't have sibling combinators to the right, \