[web-animations-1] Add repeatable list concept from CSS Transitions

Author: birtles

web-animations-1/Overview.bs

@@ -3025,7 +3025,27 @@ in each property's property definition table:
     If the number of components or the types of corresponding components
     do not match,
     or if any component value uses [=discrete=] animation,
-    the property value as a whole falls back to [=discrete=].
+    then the property values combine as [=discrete=].
+
+  <dt><dfn>repeatable list</dfn>
+  <dd>
+    Same as <a>by computed value</a>
+    except that if the two lists have differing numbers of items,
+    they are first repeated to the least common multiple number of items.
+    Each item is then combined <a>by computed value</a>.
+    If a pair of values cannot be combined
+    or if any component value uses [=discrete=] animation,
+    then the property values combine as [=discrete=].
+
+    Note: The repeatable list concept ensures that
+    a list that is conceptually repeated to a certain length
+    (as 'background-origin' is repeated to the length of
+    the 'background-image' list)
+    or repeated infinitely
+    will smoothly transition between any values,
+    and so that the computed value
+    will properly represent the result
+    (and potentially be inherited correctly).
 
   <dt>(See prose)
   <dd>
@@ -3035,8 +3055,6 @@ in each property's property definition table:
     for that property.
 </dl>
 
-<!-- TODO: Define repeatable lists -->
-
 <h4 id=custom-properties>
 Custom Properties</h4>