--- a/testing/web-platform/tests/webdriver/tests/support/fixtures.py
+++ b/testing/web-platform/tests/webdriver/tests/support/fixtures.py
@@ -14,16 +14,17 @@ from tests.support.wait import wait
 
 default_host = "http://127.0.0.1"
 default_port = "4444"
 
 default_script_timeout = 30
 default_page_load_timeout = 300
 default_implicit_wait_timeout = 0
 
+default_window_size = (800, 600)
 
 _current_session = None
 _custom_session = False
 
 
 def ignore_exceptions(f):
     def inner(*args, **kwargs):
         try:
@@ -67,17 +68,17 @@ def cleanup_session(session):
 
     @ignore_exceptions
     def _restore_window_state(session):
         """Reset window to an acceptable size.
 
         This also includes bringing it out of maximized, minimized,
         or fullscreened state.
         """
-        session.window.size = (800, 600)
+        session.window.size = default_window_size
 
     @ignore_exceptions
     def _restore_windows(session):
         """Close superfluous windows opened by the test.
 
         It will not end the session implicitly by closing the last window.
         """
         current_window = session.window_handle
@@ -208,16 +209,19 @@ def session(capabilities, configuration,
             configuration["port"],
             capabilities=caps)
     try:
         _current_session.start()
     except webdriver.error.SessionNotCreatedException:
         if not _current_session.session_id:
             raise
 
+    # Enforce a fixed default window size
+    _current_session.window.size = default_window_size
+
     yield _current_session
 
     cleanup_session(_current_session)
 
 
 def current_session():
     return _current_session