[css-nav-1] Modify Overview section

Author: Jihye Hong

css-nav-1/Overview.bs

@@ -117,16 +117,16 @@ This ability to move around the page directionally is called <dfn lt="spatial na
 <a>Spatial navigation</a> can be useful for a web page built using a grid-like layout,
 or other predominantly non linear layouts.
 The figure below represents a photo gallery arranged in a grid layout.
-If the user presses the <code class=key>Tab</code> key to move focus around the images, 
+If the user presses the <code class=key>Tab</code> key to move focus around the images,
 they need to press the key many times to reach the desired image element.
 
 <figure>
     <img alt="When elements are laid out in a grid pattern, spatial navigation makes it much easier to predict and control where focus should move to." src="images/gallery-app.png" style="width: 500px;"/>
     <figcaption>Photo gallery application example using a grid layout</figcaption>
 </figure>
 
-Also, <a>spatial navigation</a> moves the focus to the predictable element for users 
-because it moves the focus among focusable elements depending on their position. 
+Also, <a>spatial navigation</a> moves the focus to the predictable element for users
+because it moves the focus among focusable elements depending on their position.
 Sometimes elements on the page aren’t arranged independently of their source order.
 Therefore unlike <a>spatial navigation</a>, sequential navigation using the <code class=key>Tab</code> key makes focus navigation unpredictable.
 
@@ -219,9 +219,7 @@ 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 by spatial navigation once they have been scrolled into view.
-Therefore, elements that cannot be scrolled into view
-(for example because they are placed at a negative offset)
-cannot be reached by spatial navigation.
+Therefore, elements that cannot be scrolled into view by default.
 
 <div class=api>
 At key points during this search for the appropriate response to the spatial navigation request,
@@ -321,10 +319,9 @@ This intentionally left for User Agents to decide.
     will use spatial navigation as their primary or exclusive navigation mechanism.
 </div>
 
-Although it is possible for User Agents
-to implement the processing model and APIs defined by the specification
-without giving any direct means to the user to trigger spatial navigation themselves,
-this specification recommends not to do so:
+Although User Agents can implement the processing model and APIs
+defined by the specification,
+this specification recommends that
 User Agents should offer a means for users to trigger spatial navigation directly,
 without having to use the APIs.
 
@@ -623,10 +620,10 @@ This event occurs when there is a valid result of <a>selecting the best candidat
 
 The User Agent <dfn lt="dispatches navbeforefocus event | dispatching navbeforefocus event">dispatches <a event>navbeforefocus</a> event</dfn>
 before spatial navigation moves the focus.
-The <a>event target</a> is the element which has focus and 
+The <a>event target</a> is the element which has focus and
 the {{NavigationEvent/relatedTarget}} is the element which is about to receive focus.
 
-If <a>navigation-override</a> is disabled in the [=node document=] of <var>eventTarget</var> for 
+If <a>navigation-override</a> is disabled in the [=node document=] of <var>eventTarget</var> for
 the <a spec=html for="/">origin</a> of the [=active document=] of the [=top-level browsing context=], this event won't be dispatched.
 
 <div class='example'>
@@ -733,10 +730,10 @@ when it cannot be scrolled further.
 </table>
 
 The User Agent <dfn lt="dispatches navnotarget event | dispatching navnotarget event">dispatches <a event>navnotarget</a> event</dfn>
-with initializing the <a>event target</a> as the element which has focus and 
+with initializing the <a>event target</a> as the element which has focus and
 the {{NavigationEvent/relatedTarget}} as <a>spatial navigation container</a> of the <a>event target</a>.
 
-If <a>navigation-override</a> is disabled in the [=node document=] of <var>eventTarget</var> for 
+If <a>navigation-override</a> is disabled in the [=node document=] of <var>eventTarget</var> for
 the <a spec=html for="/">origin</a> of the [=active document=] of the [=top-level browsing context=], this event won't be dispatched.
 
 <div class='example'>
@@ -1024,9 +1021,9 @@ To run the <dfn>spatial navigation steps</dfn> in <var>direction</var>, do the f
                 <span class=cssapi>if computed value of the 'spatial-navigation-action' property on <var>eventTarget</var>
                 is ''spatial-navigation-action/focus'' or to <code>true</code> otherwise.
                 </span>
-            2.  
+            2.
                 <ul>
-                    <span class=cssapi><li>If 
+                    <span class=cssapi><li>If
                         the computed value of the 'spatial-navigation-action' property on <var>eventTarget</var> is not ''spatial-navigation-action/focus''
                         and <var>eventTarget</var> <a>can be manually scrolled</a>,
                         then <a>directionally scroll the element</a> <var>eventTarget</var> in <var>direction</var> and return.</li></span>
@@ -1204,7 +1201,7 @@ run the following steps:
                 * bottom edge is above the bottom edge of <var>searchOrigin</var>'s <a>boundary box</a> if <var>dir</var> is {{SpatialNavigationDirection/up}}
                 * right edge is left of the right edge of <var>searchOrigin</var>'s <a>boundary box</a> if <var>dir</var> is {{SpatialNavigationDirection/left}}
                 * left edge is right of the left edge of <var>searchOrigin</var>'s <a>boundary box</a> if <var>dir</var> is {{SpatialNavigationDirection/right}}
-                
+
                 NOTE: More detail condition about how the element is overlapped with the search origin affects the sequence of focus movement.
                 The sequence of focus movement is related to UX, so it depends on the UA-defined mechanism.