[css-ui] Hide proprietary appearance values #3968

Author: jugglinmike

css-ui-4/Overview.bs

@@ -1800,7 +1800,7 @@ Value: ''appearance/none'' | ''auto'' | ''button'' | ''textfield'' | ''menulist-
 Initial: none
 Applies To: all elements
 Inherited: no
-Computed value: specified keyword
+Computed value: See below
 Animation type: discrete
 </pre>
 
@@ -1888,6 +1888,10 @@ Animation type: discrete
 
 </dl>
 
+The computed value is the specified value unless it is not included in the set
+of values defined by this specification. In that case, it is the value
+''appearance/auto''.
+
 <div class=example>
 	An author wanting to customize the look and feel of check boxes in HTML could use the following:
 	<pre><code class="lang-css">