Add case where offset is in the worng form

Author: majido

scroll-animations-1/Overview.bs

@@ -492,21 +492,24 @@ The procedure to <dfn>resolve a container-based offset</dfn> given
     first matching condition from the following:
     <div class="switch">
 
-    :   <var>offset</var> is <code>auto</code>
+    :   If <var>offset</var> is <code>auto</code>:
     ::  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 {{CSSNumberish}} and matches <<number>>
+    :   If <var>offset</var> is a {{CSSNumberish}} and matches <<number>>:
     ::  The distance indicated by the value in <a value>pixel unit</a> along
         {{source}}'s scroll range in {{orientation}}.
 
 
-    :   <var>offset</var> is a {{CSSNumberish}} and matches
+    :   If <var>offset</var> is a {{CSSNumberish}} and matches:
         <<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>>.
 
+    :   Otherwise,
+    ::  null.
+
     </div>
 
 Note: The scroll range of an element is the range defined by its minimum and