Unknown options in extension manifest file are silently ignored

chrome/browser/resources/extensions/extensions.css

 /* Copyright (c) 2012 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. */
 
 html.loading * {
   -webkit-transition-duration: 0 !important;
 }
 
 /* Developer mode */
 
@@ skipping 26 matching lines @@
 }
 
 #extension-settings:not(.dev-mode) .developer-extras {
   display: none;
 }
 
 .developer-extras > div {
   margin: 5px 0;
 }
 
+.unrecognized-keys {
+  color: red;
+}
+
 #extension-settings #page-header {
   /* These values match the .page values. */
   -webkit-margin-end: 24px;
   min-width: 576px;
 }
 
 /* Contents */
 
 #extension-settings {
   max-width: 738px;
@@ skipping 164 matching lines @@
   right: auto;
 }
 
 .extension-list-item:not(:hover) .trash:not(:focus) {
   opacity: 0;
 }
 
 .extension-list-item-wrapper.may-not-disable .trash {
   visibility: hidden;
 }