Remove tabindex's synthetic click event feature

Other than through a feature called “spatial navigation”, currently not
enabled by default in any shipping browser, this behavior is not
implemented and at this point it’s highly likely shipping it would
break compatibility with deployed content as discussed in #1952.

Removing it to reflect reality, but as we expand the features of custom
elements, exposing “activation behavior” somehow should be high on the
list as the effective default is detrimental to keyboard-only users.

Closes #1952.

Author: annevk

source

@@ -72205,11 +72205,6 @@ END:VCARD</pre>
 
     </ul>
 
-    <p class="note">One valid reason to ignore the platform conventions and always allow an element
-    to be focused (by setting its <span>tabindex focus flag</span>) would be if the user's only
-    mechanism for activating an element is through a keyboard action that triggers the focused
-    element.</p>
-
    </dd>
 
    <dt id="negative-tabindex">If the value is a negative integer</dt>
@@ -72285,15 +72280,6 @@ END:VCARD</pre>
 
   </dl>
 
-  <p>An element that has its <span>tabindex focus flag</span> set but does not otherwise have an
-  <span>activation behavior</span> defined has an <span>activation behavior</span> that does
-  nothing.</p>
-
-  <p class="note">This means that an element that is only focusable because of its <code
-  data-x="attr-tabindex">tabindex</code> attribute will fire a <code
-  data-x="event-click">click</code> event in response to a non-mouse activation (e.g. hitting the
-  "enter" key while the element is <span>focused</span>).</p>
-
   <p>An element with the <code data-x="attr-tabindex">tabindex</code> attribute specified is
   <span>interactive content</span>.</p>