ChromeDriver should be able to focus on a frame using its frame element,

chrome/test/webdriver/commands/find_element_commands.cc

 // Copyright (c) 2011 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "chrome/test/webdriver/commands/find_element_commands.h"
 
 #include "base/values.h"
 #include "chrome/test/webdriver/commands/response.h"
 #include "chrome/test/webdriver/error_codes.h"
 #include "chrome/test/webdriver/session.h"
@@ skipping 19 matching lines @@
   if (!GetStringParameter("using", &locator) ||
       !GetStringParameter("value", &query)) {
     SET_WEBDRIVER_ERROR(response,
         "Request is missing required 'using' and/or 'value' data", kBadRequest);
     return;
   }
 
   // TODO(jmikhail): The findElement(s) atom should handle this conversion.
   if (locator == "class name") {
     locator = LocatorType::kClassName;
+  } else if (locator == "css selector") {
+    locator = LocatorType::kCss;
   } else if (locator == "link text") {
     locator = LocatorType::kLinkText;
   } else if (locator == "partial link text") {
     locator = LocatorType::kPartialLinkText;
   } else if (locator == "tag name") {
     locator = LocatorType::kTagName;
   }
   // The other locators do not need conversion. If the client supplied an
   // invalid locator, let it fail in the atom.
 
@@ skipping 16 matching lines @@
       ListValue* element_list = new ListValue();
       for (size_t i = 0; i < elements.size(); ++i)
         element_list->Append(elements[i].ToValue());
       response->SetValue(element_list);
     }
   }
   response->SetStatus(code);
 }
 
 }  // namespace webdriver