diff --git a/components/style/stylist.rs b/components/style/stylist.rs
index 18e5fb4c7d1..56f7f4b2fea 100644
--- a/components/style/stylist.rs
+++ b/components/style/stylist.rs
@@ -77,6 +77,9 @@ impl UserAgentCascadeDataCache {
         }
     }
 
+    // FIXME(emilio): This may need to be keyed on quirks-mode too, though there
+    // aren't class / id selectors on those sheets, usually, so it's probably
+    // ok...
     fn lookup<'a, I, S>(
         &'a mut self,
         sheets: I,
@@ -1146,13 +1149,11 @@ impl Stylist {
 
     /// Sets the quirks mode of the document.
     pub fn set_quirks_mode(&mut self, quirks_mode: QuirksMode) {
-        // FIXME(emilio): We don't seem to change the quirks mode dynamically
-        // during multiple layout passes, but this is totally bogus, in the
-        // sense that it's updated asynchronously.
-        //
-        // This should probably be an argument to `update`, and use the quirks
-        // mode info in the `SharedLayoutContext`.
+        if self.quirks_mode == quirks_mode {
+            return;
+        }
         self.quirks_mode = quirks_mode;
+        self.force_stylesheet_origins_dirty(OriginSet::all());
     }
 
     /// Returns the applicable CSS declarations for the given element.
diff --git a/ports/geckolib/glue.rs b/ports/geckolib/glue.rs
index ec88e40fe83..d5079bf293f 100644
--- a/ports/geckolib/glue.rs
+++ b/ports/geckolib/glue.rs
@@ -2450,19 +2450,12 @@ pub extern "C" fn Servo_StyleSet_Drop(data: RawServoStyleSetOwned) {
     let _ = data.into_box::<PerDocumentStyleData>();
 }
 
-
-/// Updating the stylesheets and redoing selector matching is always happens
-/// before the document element is inserted. Therefore we don't need to call
-/// `force_dirty` here.
 #[no_mangle]
-pub extern "C" fn Servo_StyleSet_CompatModeChanged(raw_data: RawServoStyleSetBorrowed) {
+pub unsafe extern "C" fn Servo_StyleSet_CompatModeChanged(raw_data: RawServoStyleSetBorrowed) {
     let mut data = PerDocumentStyleData::from_ffi(raw_data).borrow_mut();
-    let quirks_mode = unsafe {
-        (*data.stylist.device().pres_context().mDocument.raw::<nsIDocument>())
-            .mCompatMode
-    };
-
-    data.stylist.set_quirks_mode(quirks_mode.into());
+    let doc =
+        &*data.stylist.device().pres_context().mDocument.raw::<nsIDocument>();
+    data.stylist.set_quirks_mode(QuirksMode::from(doc.mCompatMode));
 }
 
 fn parse_property_into<R>(