[css-pseudo-4] Add UA stylesheet for spelling and grammar errors

This (together with the 2 previous commits) solves w3c#1828

Author: frivoal

css-pseudo-4/Overview.bs

@@ -450,6 +450,13 @@ Selecting Highlighted Content: the ''::selection'',  ''::inactive-selection'', '
       a portion of text that has been flagged by the user agent as grammatically incorrect.
   </dl>
 
+
+    The following addition is made to the default UA stylesheet:
+	<pre><code class="lang-css">
+		::spelling-error { text-decoration-line: spelling-error; }
+		::grammar-error { text-decoration-line: grammar-error; }
+	</code></pre>
+
   Note: A future level of CSS may introduce ways to create
   custom highlight pseudo-elements.
 
@@ -915,6 +922,7 @@ Changes</h2>
     <li>Defined handling of text decorations on ''::selection'' to match IE's more sensible behavior.
     <li>Links to pseudo-element OM design discussions.
     <li>Various minor clarifications.
+    <li>Use ''spelling-error'' and ''grammar-error'' with ''::spelling-error'' and ''::grammar-error'' in the UA stylesheet.
   </ul>
 
 <h2 class="no-num" id="acknowledgements">