--- a/devtools/shared/css/generated/mach_commands.py
+++ b/devtools/shared/css/generated/mach_commands.py
@@ -107,18 +107,18 @@ class MachCommands(MachCommandBase):
 
     def output_template(self, substitutions):
         """Output a the properties-db.js from a template."""
         js_template_path = resolve_path(self.topsrcdir,
             'devtools/shared/css/generated/properties-db.js.in')
         destination_path = resolve_path(self.topsrcdir,
             'devtools/shared/css/generated/properties-db.js')
 
-        with open(js_template_path, 'r') as handle:
+        with open(js_template_path, 'rb') as handle:
             js_template = handle.read()
 
         preamble = '/* THIS IS AN AUTOGENERATED FILE.  DO NOT EDIT */\n\n'
         contents = string.Template(js_template).substitute(substitutions)
 
-        with open(destination_path, 'w') as destination:
+        with open(destination_path, 'wb') as destination:
             destination.write(preamble + contents)
 
         print('The database was successfully generated at ' + destination_path)