--- a/layout/reftests/usercss/reftest.list
+++ b/layout/reftests/usercss/reftest.list
@@ -1,4 +1,4 @@
 == usercss.html usercss-ref.html
-random-if(!browserIsRemote&&(stylo||styloVsGecko)) == usercss-uppercase.html usercss-ref.html
+== usercss-uppercase.html usercss-ref.html
 == usercss-xbl.html usercss-ref.html
 == usercss-moz-document.html usercss-moz-document-ref.html

--- a/layout/style/ServoStyleSet.cpp
+++ b/layout/style/ServoStyleSet.cpp
@@ -1310,16 +1310,17 @@ ServoStyleSet::ClearCachedStyleData()
   ClearNonInheritingStyleContexts();
   Servo_StyleSet_RebuildCachedData(mRawSet.get());
 }
 
 void
 ServoStyleSet::CompatibilityModeChanged()
 {
   Servo_StyleSet_CompatModeChanged(mRawSet.get());
+  SetStylistStyleSheetsDirty();
 }
 
 already_AddRefed<ServoStyleContext>
 ServoStyleSet::ResolveServoStyle(Element* aElement)
 {
   RefPtr<ServoStyleContext> result =
     Servo_ResolveStyle(aElement, mRawSet.get()).Consume();
   return result.forget();

--- a/servo/components/style/stylist.rs
+++ b/servo/components/style/stylist.rs
@@ -1141,23 +1141,21 @@ impl Stylist {
 
     /// Returns the Quirks Mode of the document.
     pub fn quirks_mode(&self) -> QuirksMode {
         self.quirks_mode
     }
 
     /// 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.
     ///
     /// This corresponds to `ElementRuleCollector` in WebKit.
     pub fn push_applicable_declarations<E, F>(
         &self,
         element: E,

--- a/servo/ports/geckolib/glue.rs
+++ b/servo/ports/geckolib/glue.rs
@@ -2438,29 +2438,22 @@ pub extern "C" fn Servo_StyleSet_Rebuild
     data.stylist.device_mut().rebuild_cached_data();
 }
 
 #[no_mangle]
 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(quirks_mode));
 }
 
 fn parse_property_into<R>(
     declarations: &mut SourcePropertyDeclaration,
     property_id: PropertyId,
     value: *const nsACString,
     data: *mut URLExtraData,
     parsing_mode: structs::ParsingMode,