--- a/testing/web-platform/tests/tools/webdriver/webdriver/client.py
+++ b/testing/web-platform/tests/tools/webdriver/webdriver/client.py
@@ -347,17 +347,18 @@ class Session(object):
                  host,
                  port,
                  url_prefix="/",
                  capabilities=None,
                  timeout=None,
                  extension=None):
         self.transport = transport.HTTPWireProtocol(
             host, port, url_prefix, timeout=timeout)
-        self.capabilities = capabilities
+        self.requested_capabilities = capabilities
+        self.capabilities = None
         self.session_id = None
         self.timeouts = None
         self.window = None
         self.find = None
         self._element_cache = {}
         self.extension = None
         self.extension_cls = extension
 
@@ -385,18 +386,18 @@ class Session(object):
         self.end()
 
     def start(self):
         if self.session_id is not None:
             return
 
         body = {}
 
-        if self.capabilities is not None:
-            body["capabilities"] = self.capabilities
+        if self.requested_capabilities is not None:
+            body["capabilities"] = self.requested_capabilities
 
         value = self.send_command("POST", "session", body=body)
         self.session_id = value["sessionId"]
         self.capabilities = value["capabilities"]
 
         if self.extension_cls:
             self.extension = self.extension_cls(self)