Define that background-size is animatable if the keywords match, to match impls. Fixes w3c#158

Author: tabatkins

css-backgrounds/Overview.html

@@ -1599,14 +1599,7 @@ <h3 id=the-background-size><span class=secno>3.9. </span>Sizing Images: the
     <tr>
      <th>Animatable:
 
-     <td>as <a
-      href="http://www.w3.org/TR/css3-transitions/#animtype-repeatable-list">repeatable
-      list</a> of <a
-      href="http://www.w3.org/TR/css3-transitions/#animtype-simple-list">simple
-      list</a> of <a
-      href="http://www.w3.org/TR/css3-transitions/#animtype-lpcalc">length,
-      percentage, or calc</a> (<span class=note>This means keyword values are
-      not animatable.</span>)
+     <td>if the keywords, if any, match: as <a href="https://www.w3.org/TR/css3-transitions/#animtype-repeatable-list">repeatable list</a> of <a href="https://www.w3.org/TR/css3-transitions/#animtype-simple-list">simple list</a> of <a href="https://www.w3.org/TR/css3-transitions/#animtype-lpcalc">length, percentage, or calc</a> for the non-keyword values
   </table>
 
   <p>Specifies the size of the background images. Where

css-backgrounds/Overview.src.html

@@ -1024,8 +1024,7 @@ <h3 id="the-background-size">Sizing Images: the 'background-size' property</h3>
       <td>as specified, but with lengths made absolute and omitted ''auto'' keywords filled in
     <tr>
       <th>Animatable:
-      <td>as <a href="https://www.w3.org/TR/css3-transitions/#animtype-repeatable-list">repeatable list</a> of <a href="https://www.w3.org/TR/css3-transitions/#animtype-simple-list">simple list</a> of <a href="https://www.w3.org/TR/css3-transitions/#animtype-lpcalc">length, percentage, or calc</a>
-      (<span class="note">This means keyword values are not animatable.</span>)
+      <td>as <a href="https://www.w3.org/TR/css3-transitions/#animtype-repeatable-list">repeatable list</a> of <a href="https://www.w3.org/TR/css3-transitions/#animtype-simple-list">simple list</a> of <a href="https://www.w3.org/TR/css3-transitions/#animtype-lpcalc">length, percentage, or calc</a> and keywords
 </table>
 
 <p>Specifies the size of the background images. Where