[css-navigation-1] Provide an initial definition of @route rules. (#13148)

This was discussed briefly in
#12594 (comment)
and this PR fills in a proposed definition of such a rule.

Author: dbaron

css-navigation-1/Overview.bs

@@ -33,7 +33,77 @@ url: https://html.spec.whatwg.org/multipage/browsing-the-web.html#has-been-revea
 	type: dfn; spec: html; text: has been revealed;
 </pre>
 
-<h2 id="condional-navigation-queries">Conditional rules for navigation queries</h2>
+<h2 id="at-route">Declaring named URL patterns: the ''@route'' rule</h2>
+
+The <dfn at-rule id="at-ruledef-route">@route</dfn> rule
+is an at-rule that associates a name with a [=URL pattern=].
+This name can be referenced in ''@navigation'' rules
+and in '':link-to()'' pseudo-classes.
+
+The syntax of the ''@route'' rule is:
+
+<pre class="prod def" nohighlight>
+@route <<dashed-ident>> {
+  [ <<pattern-descriptors>> | <<init-descriptors>> ]
+}
+</pre>
+
+based on the following definitions:
+
+<pre class="prod def" dfn-type="type" nohighlight>
+<dfn><<pattern-descriptors>></dfn> = ;* <<pattern-descriptor>> ;*
+<dfn><<pattern-descriptor>></dfn> = pattern : <<url-pattern()>>
+<dfn><<init-descriptors>></dfn> = ;* <<init-descriptor>> [ ;+ <<init-descriptor>> ]* ;*
+<dfn><<init-descriptor>></dfn> = <<init-descriptor-name>> : <<string>>
+<dfn><<init-descriptor-name>></dfn> = protocol | username | password | hostname | port
+                         pathname | search | hash | baseURL
+</pre>
+
+This associates an author-defined keyword with a URL pattern,
+so that any URL that matches one of the URL patterns
+matches the route named by the keyword.
+
+The ''@route'' rule can be defined in one of two ways:
+
+: with the <code>pattern</code> descriptor
+:: in this case the URL pattern represented is
+	the one represented by the <<url-pattern()>> function
+        given as the descriptor's value.
+: with the other descriptors named by <<init-descriptor-name>>
+:: In this case the URL pattern represented is the result of invoking
+        [=URL pattern/create|create a URL pattern=] given
+	<var>input</var> as {{URLPatternInit}}
+        constructed from the descriptors and their values;
+	if a <code>baseURL</code> descriptor is not given then one is created from
+	the [=style resource base URL=] of the rule.
+
+ISSUE: Should this use <<dashed-ident>>, <<custom-ident>>, or <<ident>>
+for the route names?
+
+ISSUE: Is there value in being able to provide a list of <<url-pattern()>>
+rather than just one?
+
+<div class="example">
+Either this rule:
+<pre highlight=css>
+@route --movie-list {
+  pattern: url-pattern("/movie-list");
+}
+</pre>
+or this rule:
+<pre highlight=css>
+@route --movie-list {
+  pathname: "/movie-list";
+}
+</pre>
+define an ''@route'' rule that associates
+the name <code>--movie-list</code>
+with the URL <code>"/movie-list"</code> resolved relative to the style sheet.
+</div>
+
+NOTE: The bracing syntax also allows for future expansion if needed.
+
+<h2 id="conditional-navigation-queries">Conditional rules for navigation queries</h2>
 
 <h3 id="at-navigation">Navigation queries: the ''@navigation'' rule</h3>
 
@@ -79,7 +149,8 @@ with <<navigation-condition>> defined as:
 	<dfn><<navigation-in-parens>></dfn> = ( <<navigation-condition>> ) | ( <<navigation-test>> ) | <<general-enclosed>>
 	<dfn><<navigation-test>></dfn> = <<navigation-location>> | <<navigation-keyword>> : <<navigation-location>>
 	<dfn><<navigation-keyword>></dfn> = at | from | to
-	<dfn><<navigation-location>></dfn> = <<url-pattern()>>
+	<dfn><<navigation-location>></dfn> = <<route-name>> | <<url-pattern()>>
+	<dfn><<route-name>></dfn> = <<dashed-ident>>
 </pre>
 
 The above grammar is purposely very loose for forwards-compatibility reasons,
@@ -230,14 +301,15 @@ as follows:
 The <dfn>navigation location URL pattern</dfn> of a <<navigation-location>>
 depends on the type of <<navigation-location>>:
 
+: <<route-name>>
+::	the URL pattern represented by the ''@route'' rule referenced by the name.
+
 : <<url-pattern()>>
 ::	The [=URL pattern=] represented by the function; see
 	[=create a URL pattern for url-pattern()=].
 
-ISSUE: There is currently only one type of <<navigation-location>>
-but we should consider adding another,
-which would be just a name that references
-a name defined in a routemap.
+ISSUE: Should it also be possible to reference
+a name defined in a routemap?
 See the
 <a href="https://github.com/WICG/declarative-partial-updates/blob/main/route-matching-explainer.md">route matching explainer</a>
 for details.
@@ -280,7 +352,8 @@ to model it after.
 
 <h2 id="link-navigation-pseudo-classes">Pseudo-classes for navigation-related links</h2>
 
-This specification defines a new '':link-to()'' functional pseudo-class
+This specification defines a new
+<dfn id="link-to-pseudo" selector>'':link-to()''</dfn> functional pseudo-class
 that matches link elements that link to a certain URL.
 
 The '':link-to()'' pseudo-class takes a single argument, a <<link-condition>>,