diff --git a/components/script/dom/webidls/CSSStyleSheet.webidl b/components/script/dom/webidls/CSSStyleSheet.webidl
index 15d1fab4809..c09f14a3e8a 100644
--- a/components/script/dom/webidls/CSSStyleSheet.webidl
+++ b/components/script/dom/webidls/CSSStyleSheet.webidl
@@ -7,6 +7,6 @@
 interface CSSStyleSheet : StyleSheet {
   // readonly attribute CSSRule? ownerRule;
   [Throws, 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] void deleteRule(unsigned long index);
 };
diff --git a/tests/wpt/metadata/css/cssom/insertRule-charset-no-index.html.ini b/tests/wpt/metadata/css/cssom/insertRule-charset-no-index.html.ini
deleted file mode 100644
index 4843a2ef2ea..00000000000
--- a/tests/wpt/metadata/css/cssom/insertRule-charset-no-index.html.ini
+++ /dev/null
@@ -1,10 +0,0 @@
-[insertRule-charset-no-index.html]
-  [inserRule with charset and omitted index argument]
-    expected: FAIL
-
-  [insertRule with charset and undefined index argument]
-    expected: FAIL
-
-  [insertRule with charset and omitted index argument]
-    expected: FAIL
-
diff --git a/tests/wpt/metadata/css/cssom/insertRule-import-no-index.html.ini b/tests/wpt/metadata/css/cssom/insertRule-import-no-index.html.ini
deleted file mode 100644
index 28fbdbc5abf..00000000000
--- a/tests/wpt/metadata/css/cssom/insertRule-import-no-index.html.ini
+++ /dev/null
@@ -1,7 +0,0 @@
-[insertRule-import-no-index.html]
-  [inserRule with import and omitted index argument]
-    expected: FAIL
-
-  [insertRule with import and omitted index argument]
-    expected: FAIL
-
diff --git a/tests/wpt/metadata/css/cssom/insertRule-namespace-no-index.html.ini b/tests/wpt/metadata/css/cssom/insertRule-namespace-no-index.html.ini
index a86f87c43bb..d8a589607a2 100644
--- a/tests/wpt/metadata/css/cssom/insertRule-namespace-no-index.html.ini
+++ b/tests/wpt/metadata/css/cssom/insertRule-namespace-no-index.html.ini
@@ -1,13 +1,3 @@
 [insertRule-namespace-no-index.html]
-  [inserRule with namespace and omitted index argument]
-    expected: FAIL
-
-  [inserRule with namespace and omitted index argument should insert import]
-    expected: FAIL
-
-  [insertRule with namespace and omitted index argument]
-    expected: FAIL
-
-  [insertRule with namespace and omitted index argument should insert import]
-    expected: FAIL
+  expected: CRASH
 
diff --git a/tests/wpt/metadata/css/cssom/insertRule-no-index.html.ini b/tests/wpt/metadata/css/cssom/insertRule-no-index.html.ini
deleted file mode 100644
index 4a5e7f79e25..00000000000
--- a/tests/wpt/metadata/css/cssom/insertRule-no-index.html.ini
+++ /dev/null
@@ -1,10 +0,0 @@
-[insertRule-no-index.html]
-  [inserRule with omitted index argument]
-    expected: FAIL
-
-  [insertRule with undefined index argument]
-    expected: FAIL
-
-  [insertRule with omitted index argument]
-    expected: FAIL
-
diff --git a/tests/wpt/metadata/css/cssom/interfaces.html.ini b/tests/wpt/metadata/css/cssom/interfaces.html.ini
index d4997bef326..d6a28f8765b 100644
--- a/tests/wpt/metadata/css/cssom/interfaces.html.ini
+++ b/tests/wpt/metadata/css/cssom/interfaces.html.ini
@@ -243,9 +243,6 @@
   [MediaList interface: calling deleteMedium(CSSOMString) on style_element.sheet.media with too few arguments must throw TypeError]
     expected: FAIL
 
-  [CSSStyleSheet interface: operation insertRule(CSSOMString, unsigned long)]
-    expected: FAIL
-
   [CSSStyleSheet interface: style_element.sheet must inherit property "ownerRule" with the proper type]
     expected: FAIL