--- a/testing/web-platform/tests/tools/webdriver/webdriver/client.py
+++ b/testing/web-platform/tests/tools/webdriver/webdriver/client.py
@@ -276,16 +276,21 @@ class Window(object):
     @command
     def maximize(self):
         return self.session.send_session_command("POST", "window/maximize")
 
     @command
     def minimize(self):
         return self.session.send_session_command("POST", "window/minimize")
 
+    @property
+    @command
+    def fullscreened(self):
+        return self.session.execute_script("return document.fullscreen")
+
     @command
     def fullscreen(self):
         return self.session.send_session_command("POST", "window/fullscreen")
 
 
 class Find(object):
     def __init__(self, session):
         self.session = session