Say that future specifications may define additional types.

Author: dbaron

css3-transitions/Overview.html

@@ -1229,6 +1229,8 @@ <h2 id=animatable-types><span class=secno>6. </span><a
    can result from cubic-bezier(). Clamping values to the allowed range is
    probably the best solution.
 
+  <p>Future specifications may define additional types that can be animated.
+
   <h2 id=animatable-properties><span class=secno>7. </span><a
    id=animatable-properties-> Animatable properties </a></h2>
   <!--

css3-transitions/Overview.src.html

@@ -1228,6 +1228,9 @@ <h2 id="animatable-types"><a id=animation-of-property-types->
       values that can result from cubic-bezier().  Clamping values to the
       allowed range is probably the best solution.</p>
 
+      <p>Future specifications may define additional types that can
+      be animated.</p>
+
       <h2 id=animatable-properties><a id=animatable-properties->
         Animatable properties
       </a></h2>