[scroll-animations-1] Use <<length-percentage>> to allow calc() (w3c#4340)

Author: majido

scroll-animations-1/Overview.bs

@@ -449,11 +449,10 @@ A <dfn>container-based offset</dfn> is a scroll timeline offset that is declared
 only in relation with the <a>scroll container</a> as specified by {{source}}.
 
 A [=container-based offset=] is provided in the {{DOMString}} form and can have
-one the following three values:
+one the following values:
 
 *   auto
-*   <<length>>
-*   <<percentage>>
+*   <<length-percentage>>
 
 
 The procedure to <dfn>resolve a container-based offset</dfn> given
@@ -467,12 +466,10 @@ The procedure to <dfn>resolve a container-based offset</dfn> given
     ::  Either the beginning or the ending of {{source}}'s scroll range
         in {{orientation}} depending on whether the offset is {{start}} or {{end}}.
 
-    :   <var>offset</var> is a <<length>>
-    ::  The absolute distance indicated by the value along {{source}}'s scroll range
-        in {{orientation}}.
-
-    :  <var>offset</var> is a <<percentage>>
-    ::  The percentage distance along {{source}}'s scroll range in {{orientation}}.
+    :   <var>offset</var> is a <<length-percentage>>
+    ::  The distance indicated by the value along {{source}}'s scroll range in {{orientation}} as
+        expressed by absolute length, a percentage, or a ''calc()'' expression that resolves to a
+        <<length>>.
 
     </div>
 
@@ -831,7 +828,7 @@ instead of document's scrolling Element
 <pre class='descdef'>
   Name: start
   For: @scroll-timeline
-  Value: auto | <<length>> | <<percentage>>
+  Value: auto | <<length-percentage>>
   Initial: auto
 </pre>
 
@@ -841,7 +838,7 @@ instead of document's scrolling Element
 <pre class='descdef'>
   Name: end
   For: @scroll-timeline
-  Value: auto | <<length>> | <<percentage>>
+  Value: auto | <<length-percentage>>
   Initial: auto
 </pre>