[css-navigation-1] Add @when/if() integration, and issue about base URLs.

(This is part of importing the history from WICG/declarative-partial-updates.)

Author: dbaron

css-navigation-1/Overview.bs

@@ -16,6 +16,10 @@ Abstract: This module contains conditional CSS rules for styling based on routes
 </pre>
 <!-- for now using Org: w3c rather than Group: csswg because the latter forces an incorrect issue tracking link, overriding Repository -->
 
+<pre class="link-defaults">
+spec:css-values-5; type:function; text:if()
+</pre>
+
 <!-- FIXME: TEMPORARILY override non-exported definition -->
 <pre class=anchors>
 url: https://html.spec.whatwg.org/multipage/nav-history-apis.html#concept-navigationtransition-from
@@ -240,9 +244,45 @@ using the steps of the <dfn>create a URL pattern for urlpattern()</dfn> algorith
 1. Let <var>baseURL</var> be the [=style resource base URL=] of
 	the rule or declaration block containing the ''urlpattern()'' function.
 
+	<div class="issue">
+	Do we really want this to be the base URL?
+	For this particular use of ''urlpattern()'',
+	it's likely more useful for the base URL
+	to be the document URL rather than the style sheet URL.
+	However, it would be very awkward for ''urlpattern()''
+	to be inconsistent with ''url()''.
+	Should we introduce <code>document-urlpattern()</code>?
+	Should we do something similar to
+	[[css-images-3#ambiguous-urls]]
+	(see <w3c/csswg-drafts#383>)?
+
 1. Return the result of [=URL pattern/create|create a URL pattern=] given
 	<var>arg</var>, <var>baseURL</var>, and an empty [=map=].
 
 NOTE:  This function requires that its argument is quoted.
 This differs from the ''url()'' function,
 which allows its argument to be quoted or unquoted.
+
+<h2 id="route-when-function">The ''@when/route()'' function for ''@when''</h2>
+
+This specification defines an additional function for the ''@when'' rule:
+
+<pre class="prod">
+<dfn for="@when" function>route()</dfn> = route( <<route-condition>> )
+</pre>
+
+The ''@when/route()'' function is associated with the boolean result that
+its contained condition is associated with.
+
+<h2 id="route-if-function">The ''if()/route()'' function for ''if()''</h2>
+
+This specification defines an additional function for the ''if()'' function's
+<<if-test>> production:
+
+<pre class="prod">
+<dfn for="if()" function>route()</dfn> = route( <<route-condition>> )
+</pre>
+
+ISSUE: This should probably have a more formal definition of the function,
+but I can't find the formal definitions of the existing ''if()'' functions
+to model it after.