--- a/testing/mochitest/runtests.py
+++ b/testing/mochitest/runtests.py
@@ -1392,16 +1392,19 @@ toolbar#nav-bar {
         if self._active_tests:
             return self._active_tests
 
         tests = []
         manifest = self.getTestManifest(options)
         if manifest:
             if options.extra_mozinfo_json:
                 mozinfo.update(options.extra_mozinfo_json)
+            if 'STYLO_FORCE_ENABLED' in os.environ:
+                mozinfo.update({'stylo': True})
+
             info = mozinfo.info
 
             # Bug 1089034 - imptest failure expectations are encoded as
             # test manifests, even though they aren't tests. This gross
             # hack causes several problems in automation including
             # throwing off the chunking numbers. Remove them manually
             # until bug 1089034 is fixed.
             def remove_imptest_failure_expectations(tests, values):