[css-pseudo-4] Add “precisely”

Author: fantasai

css-pseudo-4/Overview.bs

@@ -500,7 +500,7 @@ Styling Highlights</h3>
 
   The outline, if supported, must be drawn
   around the union of the active portions of the <a>highlight overlay</a>
-  along the boundaries of those portions
+  precisely along the boundaries of those portions
   and not between congruent parts.
 
   ISSUE: Does anyone support 'outline' here? Is it wanted? Should we remove it from this list?