--- a/config/rules.mk
+++ b/config/rules.mk
@@ -1443,16 +1443,18 @@ pp_target_results = $(foreach file,$($(1
 )
 
 PP_TARGETS_ALL_RESULTS := $(sort $(foreach tier,$(PP_TARGETS_TIERS),$(PP_TARGETS_RESULTS_$(tier))))
 $(PP_TARGETS_ALL_RESULTS):
 	$(if $(filter-out $(notdir $@),$(notdir $(<:.in=))),$(error Looks like $@ has an unexpected dependency on $< which breaks PP_TARGETS))
 	$(RM) '$@'
 	$(call py_action,preprocessor,--depend $(MDDEPDIR)/$(@F).pp $(PP_TARGET_FLAGS) $(DEFINES) $(ACDEFINES) $(MOZ_DEBUG_DEFINES) '$<' -o '$@')
 
+$(filter %.css,$(PP_TARGETS_ALL_RESULTS)): PP_TARGET_FLAGS+=--marker %
+
 # The depfile is based on the filename, and we don't want conflicts. So check
 # there's only one occurrence of any given filename in PP_TARGETS_ALL_RESULTS.
 PP_TARGETS_ALL_RESULT_NAMES := $(notdir $(PP_TARGETS_ALL_RESULTS))
 $(foreach file,$(sort $(PP_TARGETS_ALL_RESULT_NAMES)), \
   $(if $(filter-out 1,$(words $(filter $(file),$(PP_TARGETS_ALL_RESULT_NAMES)))), \
     $(error Multiple preprocessing rules are creating a $(file) file) \
   ) \
 )

--- a/python/mozbuild/mozbuild/backend/fastermake.py
+++ b/python/mozbuild/mozbuild/backend/fastermake.py
@@ -42,16 +42,18 @@ class FasterMakeBackend(CommonBackend):
         self._has_xpidl = False
 
     def _add_preprocess(self, obj, path, dest, target=None, **kwargs):
         if target is None:
             target = mozpath.basename(path)
             # This matches what PP_TARGETS do in config/rules.
             if target.endswith('.in'):
                 target = target[:-3]
+        if target.endswith('.css'):
+            kwargs['marker'] = '%'
         depfile = mozpath.join(
             self.environment.topobjdir, 'faster', '.deps',
             mozpath.join(obj.install_target, dest, target).replace('/', '_'))
         self._install_manifests[obj.install_target].add_preprocess(
             mozpath.join(obj.srcdir, path),
             mozpath.join(dest, target),
             depfile,
             **kwargs)
@@ -184,26 +186,22 @@ class FasterMakeBackend(CommonBackend):
                         # it, but it's how it works in the recursive make,
                         # not that anything relies on that, but it's simpler.
                         src = mozpath.join(obj.objdir, e.source)
                     self._dependencies['install-%s' % install_target] \
                         .append(mozpath.relpath(
                         src, self.environment.topobjdir))
 
                 if e.preprocess:
-                    kwargs = {}
-                    if src.endswith('.css'):
-                        kwargs['marker'] = '%'
                     self._add_preprocess(
                         obj,
                         src,
                         mozpath.join(jarinfo.name, mozpath.dirname(e.output)),
                         mozpath.basename(e.output),
-                        defines=defines,
-                        **kwargs)
+                        defines=defines)
                 else:
                     self._install_manifests[install_target].add_symlink(
                         src,
                         mozpath.join(jarinfo.name, e.output))
 
             manifest = mozpath.normpath(mozpath.join(install_target,
                                                      jarinfo.name))
             manifest += '.manifest'

--- a/python/mozbuild/mozbuild/test/backend/data/build/app/moz.build
+++ b/python/mozbuild/mozbuild/test/backend/data/build/app/moz.build
@@ -21,16 +21,17 @@ FINAL_TARGET_FILES += [
 ]
 
 FINAL_TARGET_FILES.child += [
     '../bar.ini',
 ]
 
 FINAL_TARGET_PP_FILES += [
     '../baz.ini',
+    '../foo.css',
 ]
 
 FINAL_TARGET_PP_FILES.child2 += [
     '../qux.ini',
 ]
 
 EXTRA_COMPONENTS += [
     '../components.manifest',

new file mode 100644
--- /dev/null
+++ b/python/mozbuild/mozbuild/test/backend/data/build/foo.css
@@ -0,0 +1,2 @@
+%filter substitution
+foo.css: FOO is @FOO@

--- a/python/mozbuild/mozbuild/test/backend/data/build/jar.mn
+++ b/python/mozbuild/mozbuild/test/backend/data/build/jar.mn
@@ -1,10 +1,11 @@
 foo.jar:
 % content bar %child/
 % content foo %
   foo.js
+* foo.css
   bar.js          (subdir/bar.js)
   qux.js          (subdir/bar.js)
 * child/hoge.js   (bar.js)
 * child/baz.jsm
 
 % override   chrome://foo/bar.svg#hello   chrome://bar/bar.svg#hello

--- a/python/mozbuild/mozbuild/test/backend/data/build/moz.build
+++ b/python/mozbuild/mozbuild/test/backend/data/build/moz.build
@@ -27,16 +27,17 @@ FINAL_TARGET_FILES.child += [
     'bar.ini',
 ]
 
 FINAL_TARGET_PP_FILES += [
     'baz.ini',
 ]
 
 FINAL_TARGET_PP_FILES.child2 += [
+    'foo.css',
     'qux.ini',
 ]
 
 EXTRA_COMPONENTS += [
     'components.manifest',
     'foo.js',
 ]
 

--- a/python/mozbuild/mozbuild/test/backend/test_build.py
+++ b/python/mozbuild/mozbuild/test/backend/test_build.py
@@ -128,30 +128,32 @@ class TestBuild(unittest.TestCase):
         result = {
             p: f.open().read().decode('utf-8')
             for p, f in FileFinder(mozpath.join(config.topobjdir, 'dist'))
         }
         self.assertTrue(len(result))
         self.assertEqual(result, {
             'bin/baz.ini': 'baz.ini: FOO is foo\n',
             'bin/child/bar.ini': 'bar.ini\n',
+            'bin/child2/foo.css': 'foo.css: FOO is foo\n',
             'bin/child2/qux.ini': 'qux.ini: BAR is not defined\n',
             'bin/chrome.manifest':
                 'manifest chrome/foo.manifest\n'
                 'manifest components/components.manifest\n',
             'bin/chrome/foo.manifest':
                 'content bar foo/child/\n'
                 'content foo foo/\n'
                 'override chrome://foo/bar.svg#hello '
                 'chrome://bar/bar.svg#hello\n',
             'bin/chrome/foo/bar.js': 'bar.js\n',
             'bin/chrome/foo/child/baz.jsm':
                 '//@line 2 "%sbaz.jsm"\nbaz.jsm: FOO is foo\n' % (test_path),
             'bin/chrome/foo/child/hoge.js':
                 '//@line 2 "%sbar.js"\nbar.js: FOO is foo\n' % (test_path),
+            'bin/chrome/foo/foo.css': 'foo.css: FOO is foo\n',
             'bin/chrome/foo/foo.js': 'foo.js\n',
             'bin/chrome/foo/qux.js': 'bar.js\n',
             'bin/components/bar.js':
                 '//@line 2 "%sbar.js"\nbar.js: FOO is foo\n' % (test_path),
             'bin/components/components.manifest':
                 'component {foo} foo.js\ncomponent {bar} bar.js\n',
             'bin/components/foo.js': 'foo.js\n',
             'bin/defaults/pref/prefs.js': 'prefs.js\n',
@@ -177,24 +179,26 @@ class TestBuild(unittest.TestCase):
                 'content foo foo/\n'
                 'override chrome://foo/bar.svg#hello '
                 'chrome://bar/bar.svg#hello\n',
             'bin/app/chrome/foo/bar.js': 'bar.js\n',
             'bin/app/chrome/foo/child/baz.jsm':
                 '//@line 2 "%sbaz.jsm"\nbaz.jsm: FOO is bar\n' % (test_path),
             'bin/app/chrome/foo/child/hoge.js':
                 '//@line 2 "%sbar.js"\nbar.js: FOO is bar\n' % (test_path),
+            'bin/app/chrome/foo/foo.css': 'foo.css: FOO is bar\n',
             'bin/app/chrome/foo/foo.js': 'foo.js\n',
             'bin/app/chrome/foo/qux.js': 'bar.js\n',
             'bin/app/components/bar.js':
                 '//@line 2 "%sbar.js"\nbar.js: FOO is bar\n' % (test_path),
             'bin/app/components/components.manifest':
                 'component {foo} foo.js\ncomponent {bar} bar.js\n',
             'bin/app/components/foo.js': 'foo.js\n',
             'bin/app/defaults/preferences/prefs.js': 'prefs.js\n',
+            'bin/app/foo.css': 'foo.css: FOO is bar\n',
             'bin/app/foo.ini': 'foo.ini\n',
             'bin/app/modules/baz.jsm':
                 '//@line 2 "%sbaz.jsm"\nbaz.jsm: FOO is bar\n' % (test_path),
             'bin/app/modules/child/bar.jsm': 'bar.jsm\n',
             'bin/app/modules/child2/qux.jsm':
                 '//@line 2 "%squx.jsm"\nqux.jsm: BAR is defined\n'
                 % (test_path),
             'bin/app/modules/foo.jsm': 'foo.jsm\n',