[css-pseudo-4] Specify what happens if different parts of the overlay have different outline styles.

Author: fantasai

css-pseudo-4/Overview.bs

@@ -502,6 +502,10 @@ Styling Highlights</h3>
   around the union of the active portions of the <a>highlight overlay</a>
   precisely along the boundaries of those portions
   and not between congruent parts.
+  The UA may use the outline styling of the nearest common ancestor
+  of any continuous or discontinuous range
+  rather than piecing together varying styles of outline
+  into a single shape.
 
   ISSUE: Does anyone support 'outline' here? Is it wanted? Should we remove it from this list?