--- a/servo/components/style/stylist.rs
+++ b/servo/components/style/stylist.rs
@@ -2012,17 +2012,23 @@ impl CascadeData {
                     let style_rule = locked.read_with(&guard);
                     self.num_declarations +=
                         style_rule.block.read_with(&guard).len();
                     for selector in &style_rule.selectors.0 {
                         self.num_selectors += 1;
 
                         let map = match selector.pseudo_element() {
                             Some(pseudo) if pseudo.is_precomputed() => {
-                                debug_assert!(selector.is_universal());
+
+                                // We don't allow anonymous boxes to use some selector other than a
+                                // universal selector.
+                                if !selector.is_universal() {
+                                    continue;
+                                }
+
                                 debug_assert!(matches!(origin, Origin::UserAgent));
 
                                 precomputed_pseudo_element_decls
                                     .as_mut()
                                     .expect("Expected precomputed declarations for the UA level")
                                     .get_or_insert_with(&pseudo.canonical(), Vec::new)
                                     .push(ApplicableDeclarationBlock::new(
                                         StyleSource::Style(locked.clone()),