diff --git a/testing/web-platform/tests/interfaces/css-typed-om.idl b/testing/web-platform/tests/interfaces/css-typed-om.idl
index 00d3f49bfe65..14e93dc7637c 100644
--- a/testing/web-platform/tests/interfaces/css-typed-om.idl
+++ b/testing/web-platform/tests/interfaces/css-typed-om.idl
@@ -23,10 +23,10 @@ interface StylePropertyMapReadOnly {
 
 [Exposed=Window]
 interface StylePropertyMap : StylePropertyMapReadOnly {
-    void set(USVString property, (CSSStyleValue or USVString)... values);
-    void append(USVString property, (CSSStyleValue or USVString)... values);
-    void delete(USVString property);
-    void clear();
+    undefined set(USVString property, (CSSStyleValue or USVString)... values);
+    undefined append(USVString property, (CSSStyleValue or USVString)... values);
+    undefined delete(USVString property);
+    undefined clear();
 };
 
 partial interface Element {