[css-backgrounds-4] Let 'box-shadow-offset: none' be animated as blank shadow

Author: SebastianZ

css-backgrounds-4/Overview.bs

@@ -902,10 +902,11 @@ Inherited: no
 Percentages: N/A
 Computed value: see individual properties
 Animation type: by computed value,
-    appending blank shadows (''transparent 0 0 0 0'')
-    with a corresponding ''box-shadow-position/inset'' keyword as needed
-    to match the longer list
-    if the shorter list is otherwise compatible with the longer one
+	letting ''box-shadow-offset/none'' create a blank shadow
+	(''transparent 0 0 0 0'') when interpolated with non-'none' values
+	and appending blank shadows with a corresponding
+	''box-shadow-position/inset'' keyword as needed to match the longer list
+	if the shorter list is otherwise compatible with the longer one
 </pre>
 
   <p>The 'box-shadow' property attaches one or more drop-shadows to the box.