--- a/testing/web-platform/tests/tools/wptrunner/wptrunner/wpttest.py
+++ b/testing/web-platform/tests/tools/wptrunner/wptrunner/wpttest.py
@@ -72,16 +72,18 @@ class RunInfo(dict):
         self["product"] = product
         if debug is not None:
             self["debug"] = debug
         elif "debug" not in self:
             # Default to release
             self["debug"] = False
         if product == "firefox" and "stylo" not in self:
             self["stylo"] = False
+        if 'STYLO_FORCE_ENABLED' in os.environ:
+            self["stylo"] = True
         if extras is not None:
             self.update(extras)
 
     def _update_mozinfo(self, metadata_root):
         """Add extra build information from a mozinfo.json file in a parent
         directory"""
         path = metadata_root
         dirs = set()