diff --git a/components/script_bindings/webidls/CSSGroupingRule.webidl b/components/script_bindings/webidls/CSSGroupingRule.webidl
index 7707f38d984..cad0b7a4dac 100644
--- a/components/script_bindings/webidls/CSSGroupingRule.webidl
+++ b/components/script_bindings/webidls/CSSGroupingRule.webidl
@@ -6,7 +6,7 @@
 [Abstract, Exposed=Window]
 interface CSSGroupingRule : CSSRule {
   [SameObject] readonly attribute CSSRuleList cssRules;
-  [Throws] unsigned long insertRule(DOMString rule, unsigned long index);
+  [Throws] unsigned long insertRule(DOMString rule, optional unsigned long index = 0);
   [Throws] undefined deleteRule(unsigned long index);
 };
 
diff --git a/tests/wpt/meta/css/cssom/CSSGroupingRule-insertRule.html.ini b/tests/wpt/meta/css/cssom/CSSGroupingRule-insertRule.html.ini
deleted file mode 100644
index a3ae84d1edb..00000000000
--- a/tests/wpt/meta/css/cssom/CSSGroupingRule-insertRule.html.ini
+++ /dev/null
@@ -1,4 +0,0 @@
-[CSSGroupingRule-insertRule.html]
-  [index not specified]
-    expected: FAIL
-
diff --git a/tests/wpt/meta/css/cssom/idlharness.html.ini b/tests/wpt/meta/css/cssom/idlharness.html.ini
index 02831f7313b..cd4dfca45b1 100644
--- a/tests/wpt/meta/css/cssom/idlharness.html.ini
+++ b/tests/wpt/meta/css/cssom/idlharness.html.ini
@@ -215,9 +215,6 @@
   [CSSRule interface: sheet.cssRules[2\].cssRules[0\] must inherit property "MARGIN_RULE" with the proper type]
     expected: FAIL
 
-  [CSSGroupingRule interface: operation insertRule(CSSOMString, optional unsigned long)]
-    expected: FAIL
-
   [CSSImportRule interface: sheet.cssRules[0\] must inherit property "styleSheet" with the proper type]
     expected: FAIL