--- a/dom/bindings/GenerateCSS2PropertiesWebIDL.py
+++ b/dom/bindings/GenerateCSS2PropertiesWebIDL.py
@@ -1,24 +1,30 @@
 # This Source Code Form is subject to the terms of the Mozilla Public
 # License, v. 2.0. If a copy of the MPL was not distributed with this file,
 # You can obtain one at http://mozilla.org/MPL/2.0/.
 
 import sys
 import string
 import argparse
+import subprocess
+import buildconfig
 
 # Generates a line of WebIDL with the given spelling of the property name
 # (whether camelCase, _underscorePrefixed, etc.) and the given array of
 # extended attributes.
 def generateLine(propName, extendedAttrs):
     return "  [%s] attribute DOMString %s;\n" % (", ".join(extendedAttrs),
                                                  propName)
-def generate(output, idlFilename):
-    propList = eval(sys.stdin.read())
+def generate(output, idlFilename, preprocessorHeader):
+    cpp = buildconfig.substs['CPP'].split()
+    cpp.append(preprocessorHeader)
+    preprocessed = subprocess.check_output(cpp)
+
+    propList = eval(preprocessed)
     props = ""
     for [name, prop, id, flags, pref, proptype] in propList:
         if "CSS_PROPERTY_INTERNAL" in flags:
             continue
         # Unfortunately, even some of the getters here are fallible
         # (e.g. on nsComputedDOMStyle).
         extendedAttrs = ["Throws", "TreatNullAs=EmptyString"]
         if pref is not "":
@@ -63,13 +69,14 @@ def generate(output, idlFilename):
     idlFile.close()
 
     output.write("/* THIS IS AN AUTOGENERATED FILE.  DO NOT EDIT */\n\n" +
                  string.Template(idlTemplate).substitute({"props": props}) + '\n')
 
 def main():
     parser = argparse.ArgumentParser()
     parser.add_argument('idlFilename', help='IDL property file template')
+    parser.add_argument('preprocessorHeader', help='Header file to pass through the preprocessor')
     args = parser.parse_args()
-    generate(sys.stdout, args.idlFilename)
+    generate(sys.stdout, args.idlFilename, args.preprocessorHeader)
 
 if __name__ == '__main__':
     main()

--- a/dom/bindings/Makefile.in
+++ b/dom/bindings/Makefile.in
@@ -22,21 +22,16 @@ css2properties_dependencies = \
   $(topsrcdir)/layout/style/nsCSSPropAliasList.h \
   $(webidl_base)/CSS2Properties.webidl.in \
   $(topsrcdir)/layout/style/PythonCSSProps.h \
   $(srcdir)/GenerateCSS2PropertiesWebIDL.py \
   $(GLOBAL_DEPS) \
   $(NULL)
 
 CSS2Properties.webidl: $(css2properties_dependencies)
-	$(CPP) $(DEFINES) $(ACDEFINES) -I$(topsrcdir)/layout/style \
-	  $(topsrcdir)/layout/style/PythonCSSProps.h | \
-	    PYTHONDONTWRITEBYTECODE=1 $(PYTHON) \
-	      $(srcdir)/GenerateCSS2PropertiesWebIDL.py \
-	      $(webidl_base)/CSS2Properties.webidl.in > $@
 
 # Most of the logic for dependencies lives inside Python so it can be
 # used by multiple build backends. We simply have rules to generate
 # and include the .pp file.
 #
 # The generated .pp file contains all the important dependencies such as
 # changes to .webidl or .py files should result in code generation being
 # performed. But we do pull in file-lists.jon to catch file additions.

--- a/dom/bindings/moz.build
+++ b/dom/bindings/moz.build
@@ -143,8 +143,17 @@ if CONFIG['MOZ_SIMPLEPUSH']:
     DEFINES['MOZ_SIMPLEPUSH'] = True
 
 PYTHON_UNIT_TESTS += [
     'mozwebidlcodegen/test/test_mozwebidlcodegen.py',
 ]
 
 if CONFIG['GNU_CXX']:
     CXXFLAGS += ['-Wno-error=shadow']
+
+if CONFIG['COMPILE_ENVIRONMENT']:
+    GENERATED_FILES += ['CSS2Properties.webidl']
+    css_props = GENERATED_FILES['CSS2Properties.webidl']
+    css_props.script = 'GenerateCSS2PropertiesWebIDL.py:generate'
+    css_props.inputs = [
+        '/dom/webidl/CSS2Properties.webidl.in',
+        '/layout/style/PythonCSSProps.h',
+    ]