diff --git a/tools/drop-css-property-duplicates.js b/tools/drop-css-property-duplicates.js
index 0060aa4e3daa..23ef4e8b82a7 100644
--- a/tools/drop-css-property-duplicates.js
+++ b/tools/drop-css-property-duplicates.js
@@ -238,7 +238,7 @@ async function dropCSSPropertyDuplicates(folder) {
     if (!spec.css) {
       continue;
     }
-    for (const dfnType of ['properties', 'values']) {
+    for (const dfnType of ['properties', 'values', 'atrules']) {
       for (const prop of spec.css[dfnType]) {
         if (!prop.value) {
           continue;