--- a/testing/web-platform/tests/interfaces/cssom.idl
+++ b/testing/web-platform/tests/interfaces/cssom.idl
@@ -12,18 +12,18 @@ typedef DOMString CSSOMString;
 // (https://github.com/tidoust/reffy-reports)
 // Source: CSS Object Model (CSSOM) (https://drafts.csswg.org/cssom/)
 
 [Exposed=Window]
 interface MediaList {
   stringifier attribute [LegacyNullToEmptyString] CSSOMString mediaText;
   readonly attribute unsigned long length;
   getter CSSOMString? item(unsigned long index);
-  void appendMedium(CSSOMString medium);
-  void deleteMedium(CSSOMString medium);
+  undefined appendMedium(CSSOMString medium);
+  undefined deleteMedium(CSSOMString medium);
 };
 
 [Exposed=Window]
 interface StyleSheet {
   readonly attribute CSSOMString type;
   readonly attribute USVString? href;
   readonly attribute (Element or ProcessingInstruction)? ownerNode;
   readonly attribute CSSStyleSheet? parentStyleSheet;
@@ -32,23 +32,23 @@ interface StyleSheet {
   attribute boolean disabled;
 };
 
 [Exposed=Window]
 interface CSSStyleSheet : StyleSheet {
   readonly attribute CSSRule? ownerRule;
   [SameObject] readonly attribute CSSRuleList cssRules;
   unsigned long insertRule(CSSOMString rule, optional unsigned long index = 0);
-  void deleteRule(unsigned long index);
+  undefined deleteRule(unsigned long index);
 };
 
 partial interface CSSStyleSheet {
   [SameObject] readonly attribute CSSRuleList rules;
   long addRule(optional DOMString selector = "undefined", optional DOMString style = "undefined", optional unsigned long index);
-  void removeRule(optional unsigned long index = 0);
+  undefined removeRule(optional unsigned long index = 0);
 };
 
 [Exposed=Window]
 interface StyleSheetList {
   getter CSSStyleSheet? item(unsigned long index);
   readonly attribute unsigned long length;
 };
 
@@ -97,17 +97,17 @@ interface CSSImportRule : CSSRule {
   [SameObject, PutForwards=mediaText] readonly attribute MediaList media;
   [SameObject] readonly attribute CSSStyleSheet styleSheet;
 };
 
 [Exposed=Window]
 interface CSSGroupingRule : CSSRule {
   [SameObject] readonly attribute CSSRuleList cssRules;
   unsigned long insertRule(CSSOMString rule, optional unsigned long index = 0);
-  void deleteRule(unsigned long index);
+  undefined deleteRule(unsigned long index);
 };
 
 [Exposed=Window]
 interface CSSPageRule : CSSGroupingRule {
            attribute CSSOMString selectorText;
   [SameObject, PutForwards=cssText] readonly attribute CSSStyleDeclaration style;
 };
 
@@ -125,17 +125,17 @@ interface CSSNamespaceRule : CSSRule {
 
 [Exposed=Window]
 interface CSSStyleDeclaration {
   [CEReactions] attribute CSSOMString cssText;
   readonly attribute unsigned long length;
   getter CSSOMString item(unsigned long index);
   CSSOMString getPropertyValue(CSSOMString property);
   CSSOMString getPropertyPriority(CSSOMString property);
-  [CEReactions] void setProperty(CSSOMString property, [LegacyNullToEmptyString] CSSOMString value, optional [LegacyNullToEmptyString] CSSOMString priority = "");
+  [CEReactions] undefined setProperty(CSSOMString property, [LegacyNullToEmptyString] CSSOMString value, optional [LegacyNullToEmptyString] CSSOMString priority = "");
   [CEReactions] CSSOMString removeProperty(CSSOMString property);
   readonly attribute CSSRule? parentRule;
   [CEReactions] attribute [LegacyNullToEmptyString] CSSOMString cssFloat;
 };
 
 interface mixin ElementCSSInlineStyle {
   [SameObject, PutForwards=cssText] readonly attribute CSSStyleDeclaration style;
 };