--- a/dom/bindings/GenerateCSS2PropertiesWebIDL.py
+++ b/dom/bindings/GenerateCSS2PropertiesWebIDL.py
@@ -5,17 +5,17 @@
 import sys
 import string
 
 propList = eval(sys.stdin.read())
 props = ""
 for [prop, pref] in propList:
     extendedAttrs = ["Throws", "TreatNullAs=EmptyString"]
     if pref is not "":
-        extendedAttrs.append("Pref=%s" % pref)
+        extendedAttrs.append('Pref="%s"' % pref)
     if not prop.startswith("Moz"):
         prop = prop[0].lower() + prop[1:]
     # Unfortunately, even some of the getters here are fallible
     # (e.g. on nsComputedDOMStyle).
     props += "  [%s] attribute DOMString %s;\n" % (", ".join(extendedAttrs),
                                                    prop)
 
 idlFile = open(sys.argv[1], "r");