diff --git a/dom/webidl/CSSStyleSheet.webidl b/dom/webidl/CSSStyleSheet.webidl
index dc88888c3..4d7079fec 100644
--- a/dom/webidl/CSSStyleSheet.webidl
+++ b/dom/webidl/CSSStyleSheet.webidl
@@ -18,4 +18,12 @@ interface CSSStyleSheet : StyleSheet {
   unsigned long insertRule(DOMString rule, optional unsigned long index = 0);
   [Throws]
   void deleteRule(unsigned long index);
+
+  // Non-standard WebKit things, see https://github.com/w3c/csswg-drafts/pull/3900.
+  [Throws, BinaryName="cssRules"]
+  readonly attribute CSSRuleList rules;
+  [Throws, BinaryName="deleteRule"]
+  void removeRule(optional unsigned long index = 0);
+  [Throws]
+  long addRule(optional DOMString selector = "undefined", optional DOMString style = "undefined", optional unsigned long index);
 };
diff --git a/layout/style/CSSStyleSheet.cpp b/layout/style/CSSStyleSheet.cpp
index 0c3a99d4d..b8089ad9a 100644
--- a/layout/style/CSSStyleSheet.cpp
+++ b/layout/style/CSSStyleSheet.cpp
@@ -1918,6 +1918,40 @@ CSSStyleSheet::InsertRule(const nsAString& aRule,
   return InsertRuleInternal(aRule, aIndex, aReturn);
 }
 
+/* TenFourFox issue 592 */
+int32_t
+CSSStyleSheet::AddRule(const nsAString& aSelector, const nsAString& aBlock,
+                       const Optional<uint32_t>& aIndex, ErrorResult& aRv)
+{
+  uint32_t index;
+  if (aIndex.WasPassed()) {
+    index = aIndex.Value();
+  } else {
+    CSSRuleList* collection = GetCssRules(aRv);
+    if (!collection) {
+      // Must have thrown.
+      return -1;
+    }
+    index = collection->Length();
+  }
+
+  nsAutoString rule;
+  rule.Append(aSelector);
+  rule.AppendLiteral(" { ");
+  if (!aBlock.IsEmpty()) {
+    rule.Append(aBlock);
+    rule.Append(' ');
+  }
+  rule.Append('}');
+
+  uint32_t retval; // unused
+  nsresult rv = InsertRuleInternal(rule, index, &retval);
+  if (NS_FAILED(rv)) aRv.Throw(rv);
+
+  // As per Microsoft documentation, always return -1.
+  return -1;
+}
+
 static bool
 RuleHasPendingChildSheet(css::Rule *cssRule)
 {
diff --git a/layout/style/CSSStyleSheet.h b/layout/style/CSSStyleSheet.h
index 6b670699a..f1fd6c7d3 100644
--- a/layout/style/CSSStyleSheet.h
+++ b/layout/style/CSSStyleSheet.h
@@ -320,6 +320,8 @@ public:
   void DeleteRule(uint32_t aIndex, ErrorResult& aRv) {
     aRv = DeleteRule(aIndex);
   }
+  int32_t AddRule(const nsAString& aSelector, const nsAString& aBlock,
+                  const dom::Optional<uint32_t>& aIndex, ErrorResult& aRv);
 
   // WebIDL miscellaneous bits
   dom::ParentObject GetParentObject() const {