diff --git a/dom/bindings/parser/WebIDL.py b/dom/bindings/parser/WebIDL.py
index 30b234f621ff..944faed6ffa0 100644
--- a/dom/bindings/parser/WebIDL.py
+++ b/dom/bindings/parser/WebIDL.py
@@ -2652,9 +2652,9 @@ class IDLAttribute(IDLInterfaceMember):
         if self.type.isDictionary():
             raise WebIDLError("An attribute cannot be of a dictionary type",
                               [self.location])
-        if self.type.isSequence():
-            raise WebIDLError("An attribute cannot be of a sequence type",
-                              [self.location])
+        if self.type.isSequence() and not self.getExtendedAttribute("Cached"):
+            raise WebIDLError("A non-cached attribute cannot be of a sequence "
+                              "type", [self.location])
         if self.type.isUnion():
             for f in self.type.unroll().flatMemberTypes:
                 if f.isDictionary():
diff --git a/dom/bindings/test/TestBindingHeader.h b/dom/bindings/test/TestBindingHeader.h
index e303bb3b4e38..034dc30b497b 100644
--- a/dom/bindings/test/TestBindingHeader.h
+++ b/dom/bindings/test/TestBindingHeader.h
@@ -340,6 +340,7 @@ public:
   void PassConsequentialInterface(IndirectlyImplementedInterface&);
 
   // Sequence types
+  void GetReadonlySequence(nsTArray<int32_t>&);
   void ReceiveSequence(nsTArray<int32_t>&);
   void ReceiveNullableSequence(Nullable< nsTArray<int32_t> >&);
   void ReceiveSequenceOfNullableInts(nsTArray< Nullable<int32_t> >&);
diff --git a/dom/bindings/test/TestCodeGen.webidl b/dom/bindings/test/TestCodeGen.webidl
index e06c14fab276..09dc2565fdc0 100644
--- a/dom/bindings/test/TestCodeGen.webidl
+++ b/dom/bindings/test/TestCodeGen.webidl
@@ -295,6 +295,8 @@ interface TestInterface {
   void passConsequentialInterface(IndirectlyImplementedInterface arg);
 
   // Sequence types
+  [Cached, Pure]
+  readonly attribute sequence<long> readonlySequence;
   sequence<long> receiveSequence();
   sequence<long>? receiveNullableSequence();
   sequence<long?> receiveSequenceOfNullableInts();
diff --git a/dom/bindings/test/TestExampleGen.webidl b/dom/bindings/test/TestExampleGen.webidl
index edfdaf8fc52a..6910b7124d72 100644
--- a/dom/bindings/test/TestExampleGen.webidl
+++ b/dom/bindings/test/TestExampleGen.webidl
@@ -191,6 +191,8 @@ interface TestExampleInterface {
   void passConsequentialInterface(IndirectlyImplementedInterface arg);
 
   // Sequence types
+  [Cached, Pure]
+  readonly attribute sequence<long> readonlySequence;
   sequence<long> receiveSequence();
   sequence<long>? receiveNullableSequence();
   sequence<long?> receiveSequenceOfNullableInts();
diff --git a/dom/bindings/test/TestJSImplGen.webidl b/dom/bindings/test/TestJSImplGen.webidl
index 4b7d18ef59de..208cf1ac41df 100644
--- a/dom/bindings/test/TestJSImplGen.webidl
+++ b/dom/bindings/test/TestJSImplGen.webidl
@@ -212,6 +212,8 @@ interface TestJSImplInterface {
   void passConsequentialInterface(IndirectlyImplementedInterface arg);
 
   // Sequence types
+  [Cached, Pure]
+  readonly attribute sequence<long> readonlySequence;
   sequence<long> receiveSequence();
   sequence<long>? receiveNullableSequence();
   sequence<long?> receiveSequenceOfNullableInts();