--- a/dom/bindings/Codegen.py
+++ b/dom/bindings/Codegen.py
@@ -4389,41 +4389,43 @@ class CGMemberJITInfo(CGThing):
     """
     def __init__(self, descriptor, member):
         self.member = member
         self.descriptor = descriptor
 
     def declare(self):
         return ""
 
-    def defineJitInfo(self, infoName, opName, infallible):
+    def defineJitInfo(self, infoName, opName, infallible, constant):
         protoID = "prototypes::id::%s" % self.descriptor.name
         depth = "PrototypeTraits<%s>::Depth" % protoID
-        failstr = "true" if infallible else "false"
+        failstr = toStringBool(infallible)
+        conststr = toStringBool(constant)
         return ("\n"
                 "const JSJitInfo %s = {\n"
                 "  %s,\n"
                 "  %s,\n"
                 "  %s,\n"
                 "  %s,  /* isInfallible. False in setters. */\n"
-                "  false  /* isConstant. Only relevant for getters. */\n"
-                "};\n" % (infoName, opName, protoID, depth, failstr))
+                "  %s  /* isConstant. Only relevant for getters. */\n"
+                "};\n" % (infoName, opName, protoID, depth, failstr, conststr))
 
     def define(self):
         if self.member.isAttr():
             getterinfo = ("%s_getterinfo" % self.member.identifier.name)
             getter = ("(JSJitPropertyOp)get_%s" % self.member.identifier.name)
             getterinfal = "infallible" in self.descriptor.getExtendedAttributes(self.member, getter=True)
             getterinfal = getterinfal and infallibleForMember(self.member, self.member.type, self.descriptor)
-            result = self.defineJitInfo(getterinfo, getter, getterinfal)
+            getterconst = self.member.getExtendedAttribute("Constant")
+            result = self.defineJitInfo(getterinfo, getter, getterinfal, getterconst)
             if not self.member.readonly or self.member.getExtendedAttribute("PutForwards") is not None:
                 setterinfo = ("%s_setterinfo" % self.member.identifier.name)
                 setter = ("(JSJitPropertyOp)set_%s" % self.member.identifier.name)
                 # Setters are always fallible, since they have to do a typed unwrap.
-                result += self.defineJitInfo(setterinfo, setter, False)
+                result += self.defineJitInfo(setterinfo, setter, False, False)
             return result
         if self.member.isMethod():
             methodinfo = ("%s_methodinfo" % self.member.identifier.name)
             name = CppKeywords.checkMethodName(self.member.identifier.name)
             # Actually a JSJitMethodOp, but JSJitPropertyOp by struct definition.
             method = ("(JSJitPropertyOp)%s" % name)
 
             # Methods are infallible if they are infallible, have no arguments
@@ -4434,17 +4436,17 @@ class CGMemberJITInfo(CGThing):
             if len(sigs) == 1:
                 # Don't handle overloading. If there's more than one signature,
                 # one of them must take arguments.
                 sig = sigs[0]
                 if len(sig[1]) == 0 and infallibleForMember(self.member, sig[0], self.descriptor):
                     # No arguments and infallible return boxing
                     methodInfal = True
 
-            result = self.defineJitInfo(methodinfo, method, methodInfal)
+            result = self.defineJitInfo(methodinfo, method, methodInfal, False)
             return result
         raise TypeError("Illegal member type to CGPropertyJITInfo")
 
 def getEnumValueName(value):
     # Some enum values can be empty strings.  Others might have weird
     # characters in them.  Deal with the former by returning "_empty",
     # deal with possible name collisions from that by throwing if the
     # enum value is actually "_empty", and throw on any value

--- a/dom/bindings/parser/WebIDL.py
+++ b/dom/bindings/parser/WebIDL.py
@@ -2138,16 +2138,19 @@ class IDLAttribute(IDLInterfaceMember):
                 raise WebIDLError("[LenientThis] is only allowed on non-static "
                                   "attributes", [attr.location, self.location])
             self.lenientThis = True
         elif identifier == "Unforgeable":
             if not self.readonly:
                 raise WebIDLError("[Unforgeable] is only allowed on readonly "
                                   "attributes", [attr.location, self.location])
             self._unforgeable = True
+        elif identifier == "Constant" and not self.readonly:
+            raise WebIDLError("[Constant] only allowed on readonly attributes",
+                              [attr.location, self.location])
         elif identifier == "PutForwards":
             if not self.readonly:
                 raise WebIDLError("[PutForwards] is only allowed on readonly "
                                   "attributes", [attr.location, self.location])
             if self.isStatic():
                 raise WebIDLError("[PutForwards] is only allowed on non-static "
                                   "attributes", [attr.location, self.location])
             if self.getExtendedAttribute("Replaceable") is not None:
@@ -2636,16 +2639,20 @@ class IDLMethod(IDLInterfaceMember, IDLS
         elif identifier == "SetterThrows":
             raise WebIDLError("Methods must not be flagged as "
                               "[SetterThrows]",
                               [attr.location, self.location])
         elif identifier == "Unforgeable":
             raise WebIDLError("Methods must not be flagged as "
                               "[Unforgeable]",
                               [attr.location, self.location])
+        elif identifier == "Constant":
+            raise WebIDLError("Methods must not be flagged as "
+                              "[Constant]",
+                              [attr.location, self.location]);
         elif identifier == "PutForwards":
             raise WebIDLError("Only attributes support [PutForwards]",
                               [attr.location, self.location])
         elif identifier == "LenientFloat":
             # This is called before we've done overload resolution
             assert len(self.signatures()) == 1
             sig = self.signatures()[0]
             if not sig[0].isVoid():

--- a/dom/webidl/ImageData.webidl
+++ b/dom/webidl/ImageData.webidl
@@ -8,10 +8,10 @@
  *
  * © Copyright 2004-2011 Apple Computer, Inc., Mozilla Foundation, and Opera Software ASA.
  * You are granted a license to use, reproduce and create derivative works of this document.
  */
 
 interface ImageData {
  readonly attribute unsigned long width;
  readonly attribute unsigned long height;
- readonly attribute Uint8ClampedArray data;
+ [Constant] readonly attribute Uint8ClampedArray data;
 };