diff --git a/components/style/properties/properties.mako.rs b/components/style/properties/properties.mako.rs
index de650d52e3c..f1ddcd53b2b 100644
--- a/components/style/properties/properties.mako.rs
+++ b/components/style/properties/properties.mako.rs
@@ -3330,7 +3330,6 @@ where
             if seen.contains(physical_longhand_id) {
                 continue
             }
-            seen.insert(physical_longhand_id);
 
             let mut declaration = match *declaration {
                 PropertyDeclaration::WithVariables(id, ref unparsed) => {
@@ -3376,6 +3375,8 @@ where
                 }
             }
 
+            seen.insert(physical_longhand_id);
+
             % if category_to_cascade_now == "early":
                 if LonghandId::FontSize == longhand_id {
                     font_size = Some(declaration.clone());