Add "user attention" definition to top-level traversable

As well as document's "fully active descendant of a top-level traversible with user attention" for callers.

Fixes #6211.

source

@@ -77263,8 +77263,19 @@ partial interface <span id="NavigatorUserActivation-partial">Navigator</span> {
   system, possibly targeted at one of its <span data-x="nav-document">active document</span>'s
   <span>descendant navigables</span>.</p>
 
-  <p class="note">System focus is lost when a browser window loses focus, but might also be lost to
-  other system widgets in the browser window such as a URL bar.</p>
+  <p>A <span>top-level traversable</span> has <dfn for="top-level traversable">user attention</dfn>
+  when its <span>system visibility state</span> is "<code data-x="">visible</code>", and it either
+  has <span>system focus</span> or user agent widgets directly related to it can receive keyboard
+  input channeled from the operating system.</p>
+  <!-- This language is borrowed from system focus -->
+
+  <p class="note">User attention is lost when a browser window loses focus, whereas system focus
+  might also be lost to other system widgets in the browser window such as a location bar.</p>
+
+  <p>A <code>Document</code> <var>d</var> is a <dfn export for="Document">fully active descendant of
+  a top-level traversible with user attention</dfn> when <var>d</var> is <span>fully active</span>
+  and <var>d</var>'s <span>node navigable</span>'s <span data-x="nav-top">top-level
+  traversable</span> has <span>user attention</span>.</p>
 
   <p>The term <dfn>focusable area</dfn> is used to refer to regions of the interface that can further become
   the target of such keyboard input. Focusable areas can be elements, parts of elements, or other regions