diff --git a/dom/bindings/Codegen.py b/dom/bindings/Codegen.py
index 9698d16084..4591d5720c 100644
--- a/dom/bindings/Codegen.py
+++ b/dom/bindings/Codegen.py
@@ -11043,6 +11043,7 @@ class CGDescriptor(CGThing):
             hasPromiseReturningMethod) = False, False, False, False, False, False
         jsonifierMethod = None
         crossOriginMethods, crossOriginGetters, crossOriginSetters = set(), set(), set()
+        unscopableNames = list()
         for n in descriptor.interface.namedConstructors:
             cgThings.append(CGClassConstructor(descriptor, n,
                                                NamedConstructorName(n)))
@@ -11055,6 +11056,9 @@ class CGDescriptor(CGThing):
             props = memberProperties(m, descriptor)
 
             if m.isMethod():
+                if m.getExtendedAttribute("Unscopable"):
+                    assert not m.isStatic()
+                    unscopableNames.append(m.identifier.name)
                 if props.isJsonifier:
                     jsonifierMethod = m
                 elif not m.isIdentifierLess() or m == descriptor.operations['Stringifier']:
@@ -11076,6 +11080,9 @@ class CGDescriptor(CGThing):
                     raise TypeError("Stringifier attributes not supported yet. "
                                     "See bug 824857.\n"
                                     "%s" % m.location)
+                if m.getExtendedAttribute("Unscopable"):
+                    assert not m.isStatic()
+                    unscopableNames.append(m.identifier.name)
                 if m.isStatic():
                     assert descriptor.interface.hasInterfaceObject()
                     cgThings.append(CGStaticGetter(descriptor, m))
@@ -11260,8 +11267,18 @@ class CGDescriptor(CGThing):
             cgThings.extend(CGClearCachedValueMethod(descriptor, m) for
                             m in clearableCachedAttrs(descriptor))
 
+        haveUnscopables = (len(unscopableNames) != 0 and
+                           descriptor.interface.hasInterfacePrototypeObject())
+        if haveUnscopables:
+            cgThings.append(
+                CGList([CGGeneric("static const char* const unscopableNames[] = {"),
+                        CGIndenter(CGList([CGGeneric('"%s"' % name) for
+                                           name in unscopableNames] +
+                                          [CGGeneric("nullptr")], ",\n")),
+                        CGGeneric("};\n")], "\n"))
+
         # CGCreateInterfaceObjectsMethod needs to come after our
-        # CGDOMJSClass, if any.
+        # CGDOMJSClass and unscopables, if any.
         cgThings.append(CGCreateInterfaceObjectsMethod(descriptor, properties))
 
         # CGGetProtoObjectMethod and CGGetConstructorObjectMethod need
diff --git a/dom/bindings/parser/WebIDL.py b/dom/bindings/parser/WebIDL.py
index 6a4674a42a..7dba781a94 100644
--- a/dom/bindings/parser/WebIDL.py
+++ b/dom/bindings/parser/WebIDL.py
@@ -3455,6 +3455,14 @@ class IDLAttribute(IDLInterfaceMember):
                                   "readonly attributes" % attr.value(),
                                   [attr.location, self.location])
             self._setDependsOn(attr.value())
+        elif identifier == "Unscopable":
+            if not attr.noArguments():
+                raise WebIDLError("[Unscopable] must take no arguments",
+                                  [attr.location])
+            if self.isStatic():
+                raise WebIDLError("[Unscopable] is only allowed on non-static "
+                                  "attributes and operations",
+                                  [attr.location, self.location])
         elif (identifier == "Pref" or
               identifier == "SetterThrows" or
               identifier == "Throws" or
@@ -4078,6 +4086,14 @@ class IDLMethod(IDLInterfaceMember, IDLScope):
                 raise WebIDLError("[DependsOn] takes an identifier",
                                   [attr.location])
             self._setDependsOn(attr.value())
+        elif identifier == "Unscopable":
+            if not attr.noArguments():
+                raise WebIDLError("[Unscopable] must take no arguments",
+                                  [attr.location])
+            if self.isStatic():
+                raise WebIDLError("[Unscopable] is only allowed on non-static "
+                                  "attributes and operations",
+                                  [attr.location, self.location])
         elif (identifier == "Throws" or
               identifier == "NewObject" or
               identifier == "ChromeOnly" or