diff --git a/components/malloc_size_of/lib.rs b/components/malloc_size_of/lib.rs
index 629cb1f0824..bb4fe1a6adc 100644
--- a/components/malloc_size_of/lib.rs
+++ b/components/malloc_size_of/lib.rs
@@ -778,6 +778,12 @@ impl<Impl: selectors::parser::SelectorImpl> MallocSizeOf
     }
 }
 
+impl MallocSizeOf for selectors::context::QuirksMode {
+    fn size_of(&self, _ops: &mut MallocSizeOfOps) -> usize {
+        0
+    }
+}
+
 impl MallocSizeOf for Void {
     #[inline]
     fn size_of(&self, _ops: &mut MallocSizeOfOps) -> usize {
diff --git a/components/style/author_styles.rs b/components/style/author_styles.rs
index 46f95e1cac9..eff64ed6c1e 100644
--- a/components/style/author_styles.rs
+++ b/components/style/author_styles.rs
@@ -30,7 +30,6 @@ where
     pub data: CascadeData,
     /// The quirks mode of the last stylesheet flush, used because XBL sucks and
     /// we should really fix it, see bug 1406875.
-    #[ignore_malloc_size_of = "XXX"]
     pub quirks_mode: QuirksMode,
 }
 
diff --git a/components/style/stylist.rs b/components/style/stylist.rs
index 045d6b01cd1..ece14e9896f 100644
--- a/components/style/stylist.rs
+++ b/components/style/stylist.rs
@@ -23,7 +23,7 @@ use crate::selector_map::{PrecomputedHashMap, PrecomputedHashSet, SelectorMap, S
 use crate::selector_parser::{PerPseudoElementMap, PseudoElement, SelectorImpl, SnapshotMap};
 use crate::shared_lock::{Locked, SharedRwLockReadGuard, StylesheetGuards};
 use crate::stylesheet_set::{DataValidity, DocumentStylesheetSet, SheetRebuildKind};
-use crate::stylesheet_set::{DocumentStylesheetFlusher, SheetCollectionFlusher, StylesheetSet};
+use crate::stylesheet_set::{DocumentStylesheetFlusher, SheetCollectionFlusher};
 use crate::stylesheets::keyframes_rule::KeyframesAnimation;
 use crate::stylesheets::viewport_rule::{self, MaybeNew, ViewportRule};
 use crate::stylesheets::StyleRule;