Refine the overview section

* Refined the overview section
* Fix several grammar mistakes

Author: Jihye Hong

css-nav-1/Overview.bs

@@ -199,32 +199,31 @@ next time spatial navigation in the same direction is requested.
 If the <a>spatial navigation focus container</a> cannot be scrolled,
 either because it is not a scrollable element
 or because it is already scrolled to the maximum in that direction,
-the User Agent will select the next <a>spatial navigation focus container</a> up the ancestry chain,
-and repeat the process of
-looking for eligible focus targets,
-selecting the best one if there's any,
-scrolling if not,
-going up the ancestry chain if it cannot scroll,
-until it has either moved focus,
-scrolled,
-or reached the root.
-
-Note: As a consequence of this processing model,
+the User Agent will select the next <a>spatial navigation focus container</a> up the ancestry chain.
+
+The process of looking for an eligible focus target and 
+if there is any, then move the focus, 
+else if the <a>spatial navigation focus container</a> can be scrolled, then scroll, 
+otherwise, going up to the next container
+is repeated until the ancestry chain reaches the root element.
+
+<div class=note>
+NOTE: As a consequence of this processing model,
 the elements that are reachable by sequential navigation
 and by spatial navigation are almost the same:
-elements that are currently outside of the viewport of a scrollable element
-can only be reached once they have been scrolled into view,
-and those that cannot be scrolled into view,
-for instance because they are placed at a negative offset,
-are out of reach.
+    * Elements that are currently outside of the viewport of a scrollable element
+        can only be reached once they have been scrolled into view
+    * Elements that cannot be scrolled into view. 
+        For instance, those are out of reach because they are placed at a negative offset.
+</div>
 
 <div class=api>
 At key points during this search for the appropriate response to the spatial navigation request,
 the User Agent will fire events.
 These enable authors to prevent the upcoming action
 (by calling {{preventDefault()}}),
 and if desired to provide an alternate action,
-such as using calling the {{HTMLOrSVGElement/focus()}} method on a different
+such as using the {{HTMLOrSVGElement/focus()}} method on a different
 element of the author's choosing.
 
 To help authors write such alternate actions,
@@ -251,7 +250,7 @@ and [[#js-api]] for details about the JavaScript API.
     </figure>
 
     On the left of figure 2, "Box 2" is focused.
-    Pressing the <code class=key>ArrowDown</code> key moves focus to
+    Pressing the <code class=key>ArrowDown</code> key moves the focus to
     "Box 3" without scrolling because "Box 3" is visible in the <a>scrollport</a> of the <a>spatial navigation focus container</a>.
 
     <figure>
@@ -880,7 +879,7 @@ The <a>navigation-override</a> [=policy-controlled feature=]</h2>
 
 	Note: This is to prevent a hostile iframe from using these events
 	in order to highjack the focus.
-	We recognize that there exists other mechanisms predating spatial navigation
+	We recognize that there exist other mechanisms predating spatial navigation
 	that malicious authors could use
 	to interfere with the user's ability to control where the focus goes.
 	Despite that, it seems worthwhile to attempt not to increase this attack surface,