Use 'User Agent' term consistently in the spec

Use 'User Agent' instead of 'UA'

Author: Jihye Hong

css-nav-1/Overview.bs

@@ -241,7 +241,7 @@ and [[#js-api]] for details about the JavaScript API.
     arranged in a scrollable element
     would be navigated when using spatial navigation.
     For the sake of keeping the description simple,
-    this example assumes a UA where spatial navigation is triggered using arrow keys.
+    this example assumes a User Agent where spatial navigation is triggered using arrow keys.
 
     <figure>
         <img alt="" src="images/spatnav-scroll-visible-1.png" style="width: 200px;"/>
@@ -321,7 +321,7 @@ User Agents should offer a means for users to trigger spatial navigation directl
 without having to use the APIs.
 
 Note: Conversely, authors should assume that spatial navigation may be triggered
-by the UA in response to user actions
+by the User Agent in response to user actions
 even if the author has not invoked any of the APIs.
 
 Regardless of the actual mechanism chosen to trigger spatial navigation,
@@ -370,7 +370,7 @@ the following requirements apply:
     (in addition to moving the focus)
     User Agents should not offer the same mechanism to trigger both spatial navigation
     and a scrolling behavior separate from spatial navigation.
-    UAs may however, offer a way for the user to switch between different modes,
+    User Agents may however, offer a way for the user to switch between different modes,
     or offer both based on different UI mechanism.
 
     <div class=example>
@@ -403,7 +403,7 @@ the same chain of events will be fired and
 the same element will be scrolled or focused.
 
 Note: Authors can use this to trigger spatial navigation
-based on a different UI mechanism than the one assigned by the UA,
+based on a different UI mechanism than the one assigned by the User Agent,
 such as mapping to different keys,
 or triggering spatial navigation from a clickable on-screen directional pad.
 or in reaction to other events than UI ones.
@@ -683,7 +683,7 @@ The <a event>navbeforefocus</a> event occurs before spatial navigation changes t
     This example shows the [[UI-EVENTS#event-order]] when pressing the <code class=key>ArrowRight</code>
     key.
     For the sake of keeping the description simple,
-    this example assumes a UA where spatial navigation is triggered using arrow keys.
+    this example assumes a User Agent where spatial navigation is triggered using arrow keys.
 
     <table class="complex data">
         <thead>
@@ -760,7 +760,7 @@ and it cannot be scrolled at the same time.
     This example shows the [[UI-EVENTS#event-order]] when pressing the <code class=key>ArrowDown</code>
     key in the situation like the following figure.
     For the sake of keeping the description simple,
-    this example assumes a UA where spatial navigation is triggered using arrow keys.
+    this example assumes a User Agent where spatial navigation is triggered using arrow keys.
 
     <figure>
         <img alt="An image about navnotarget" src="images/navnotarget-example-1.png" style="width: 200px;"/>
@@ -946,7 +946,7 @@ The user agent may set it when the user indicates that it should be moved.
 Note: For example, the user agent could set it to the position of the user's click if the user clicks on the document contents,
 and unset when the focus is moved (by spatial navigation or any other means).
 
-If the UA sets both a <a>spatial navigation starting point</a> and a <a>sequential focus navigation starting point</a>,
+If the User Agent sets both a <a>spatial navigation starting point</a> and a <a>sequential focus navigation starting point</a>,
 they must not be set differently.
 
 <div algorithm="to run the spatial navigation steps">