--- a/servo/components/style/stylist.rs
+++ b/servo/components/style/stylist.rs
@@ -235,16 +235,20 @@ impl DocumentCascadeData {
         origin: Origin,
         cascade_data: &mut CascadeData,
     ) -> Result<(), FailedAllocationError>
     where
         S: StylesheetInDocument + ToMediaListKey + PartialEq + 'static,
     {
         debug_assert_ne!(origin, Origin::UserAgent);
 
+        if !flusher.origin_dirty(origin) {
+            return Ok(());
+        }
+
         let validity = flusher.data_validity(origin);
 
         match validity {
             DataValidity::Valid => {},
             DataValidity::CascadeInvalid => cascade_data.clear_cascade_data(),
             DataValidity::FullyInvalid => cascade_data.clear(),
         }