MD Settings: label and describe a bunch of subpage/open-in-new buttons

chrome/browser/resources/settings/appearance_page/appearance_fonts_page.html

 <link rel="import" href="chrome://resources/cr_elements/cr_slider/cr_slider.html">
 <link rel="import" href="chrome://resources/html/i18n_behavior.html">
 <link rel="import" href="chrome://resources/html/polymer.html">
 <link rel="import" href="chrome://resources/html/web_ui_listener_behavior.html">
 <link rel="import" href="chrome://resources/polymer/v1_0/paper-icon-button/paper-icon-button-light.html">
 <link rel="import" href="fonts_browser_proxy.html">
 <link rel="import" href="../controls/settings_dropdown_menu.html">
 <link rel="import" href="../i18n_setup.html">
 <link rel="import" href="../settings_shared_css.html">
 
@@ skipping 96 matching lines @@
               $i18n{quickBrownFox}
             </span>
           </div>
         </div>
       </div>
     </div>
     <template is="dom-if" if="[[!isGuest_]]">
       <div class="settings-box two-line" id="advancedButton"
           on-tap="openAdvancedExtension_" actionable>
         <div class="start">
-          <div>$i18n{advancedFontSettings}</div>
+          $i18n{advancedFontSettings}
           <div class="secondary" id="advancedButtonSublabel">
             [[advancedExtensionSublabel_]]
           </div>
         </div>
-        <button class="icon-external" is="paper-icon-button-light"></button>
+        <button class="icon-external" is="paper-icon-button-light"
+            aria-label="$i18n{advancedFontSettings}"
+            aria-describedby="advancedButtonSublabel"></button>
       </div>
     </template>
   </template>
   <script src="appearance_fonts_page.js"></script>
 </dom-module>