[css-nav-1] Clear description about updating search origin

Make the process of updating search origin more clear,
depending on the discussion in w3c#4309

Author: Jihye Hong

css-nav-1/Overview.bs

@@ -1097,17 +1097,17 @@ run the following steps:
     then return it.
 3. Otherwise, return <var>searchOrigin</var>.
 
-If the status of <var>old focus target</var> changed not by moving the focus,
+If the status of <var>focus target</var> changed not by moving the focus,
 <dfn lt="update the search origin | updating the search origin">update the <a>search origin</a></dfn> as following:
-1. If <var>old focus target</var> is
+1. If <var>focus target</var> becomes
     <a>actually disabled</a> or
     <a>expressly inert</a> or
     not <a>being rendered</a>,
-    then let <var>searchOrigin</var> be the [=boundary box=] of <var>old focus target</var>.
-2. If <var>old focus target</var> is <a>removed</a>,
-    then let <var>searchOrigin</var> be the [=boundary box=] of <var>old focus target</var> with the position of <var>old focus target</var> when it existed.
-3. If <var>old focus target</var> is completely off-screen,
-    then let <var>searchOrigin</var> be the viewport of <var>old focus target</var>'s <a>spatial navigation container</a>.
+    then let <var>searchOrigin</var> be the [=boundary box=] of <var>focus target</var>.
+2. If <var>focus target</var> is <a>removed</a>,
+    then let <var>searchOrigin</var> be the [=boundary box=] of <var>focus target</var> with the position of <var>focus target</var> when it existed.
+3. If <var>focus target</var> is completely off-screen,
+    then let <var>searchOrigin</var> be the viewport of <var>focus target</var>'s nearest visible <a>spatial navigation container</a>.
 
 </div>