[css-ui] Resolve Issue 29 nav-* directional properties prose fixes, ID selector syntax,unmatched behavior

Author: tantek

css-ui/Overview.bs

@@ -1546,13 +1546,16 @@ Note: The 'nav-index' property is at risk.
 <dt>auto</dt>
 <dd>The user agent automatically determines which element to navigate the focus to in response to directional navigational input.</dd>
 <dt>&lt;id&gt;</dt>
-<dd><p>The &lt;id&gt; value consists of a ''#'' character followed by an identifier,
-similar to a fragment identifier in a URL.
-It indicates the element to which the focus is navigated to in response to directional navigation input respective to the specific property.
+<dd><p>The &lt;id&gt; value is an ID selector [[SELECT]].
+In response to directional navigation input corresponding to the property,
+the focus is navigated to the first element in tree order matching the selector.
 
-If the &lt;id&gt; refers to the currently focused element,
+If this refers to the currently focused element,
 the directional navigation input respective to the nav- property is ignored &mdash;
 there is no need to refocus the same element.
+
+If no element matches the selector,
+the user agent automatically determines which element to navigate the focus to.
 </dd>
 <dt>&lt;target-name&gt;</dt>
 <dd>

css-ui/Overview.html

@@ -2195,13 +2195,16 @@ <h4 class="heading settled" data-level=8.2.2 id=nav-dir><span class=secno>8.2.2.
 <dt>auto</dt>
 <dd>The user agent automatically determines which element to navigate the focus to in response to directional navigational input.</dd>
 <dt>&lt;id&gt;</dt>
-<dd><p>The &lt;id&gt; value consists of a <span class=css>#</span> character followed by an identifier,
-similar to a fragment identifier in a URL.
-It indicates the element to which the focus is navigated to in response to directional navigation input respective to the specific property.
+<dd><p>The &lt;id&gt; value is an ID selector <a data-biblio-type=informative data-link-type=biblio href=#biblio-select title=SELECT>[SELECT]</a>.
+In response to directional navigation input corresponding to the property,
+the focus is navigated to the first element in tree order matching the selector.
 
-<p>If the &lt;id&gt; refers to the currently focused element,
+<p>If this refers to the currently focused element,
 the directional navigation input respective to the nav- property is ignored —
 there is no need to refocus the same element.</p>
+
+<p>If no element matches the selector,
+the user agent automatically determines which element to navigate the focus to.</p>
 </dd>
 <dt>&lt;target-name&gt;</dt>
 <dd>