--- a/testing/web-platform/tests/tools/wptrunner/wptrunner/wptcommandline.py
+++ b/testing/web-platform/tests/tools/wptrunner/wptrunner/wptcommandline.py
@@ -391,16 +391,20 @@ def check_args(kwargs):
     if kwargs['extra_prefs']:
         missing = any('=' not in prefarg for prefarg in kwargs['extra_prefs'])
         if missing:
             print >> sys.stderr, "Preferences via --setpref must be in key=value format"
             sys.exit(1)
         kwargs['extra_prefs'] = [tuple(prefarg.split('=', 1)) for prefarg in
                                  kwargs['extra_prefs']]
 
+    if kwargs["reftest_internal"] is None:
+        # Default to the internal reftest implementation on Linux and OSX
+        kwargs["reftest_internal"] = sys.platform.startswith("linux") or sys.platform.startswith("darwin")
+
     return kwargs
 
 
 def check_args_update(kwargs):
     set_from_config(kwargs)
 
     if kwargs["product"] is None:
         kwargs["product"] = "firefox"