[css-values-4] Formally provide <url()> and <src()> productions. Spec the <url-token> behavior in the grammar. w3c/fxtf-drafts#508

Author: tabatkins

css-values-4/Overview.bs

@@ -849,8 +849,10 @@ Resource Locators: the <<url>> type</h3>
 	The syntax of <<url>> is:
 
 	<pre class="prod">
-		<dfn id="url-value">&lt;url></dfn> = url( <<string>> <<url-modifier>>* ) |
-		        src( <<string>> <<url-modifier>>* )
+		<dfn id="url-value">&lt;url></dfn> = <<url()>> | <<src()>>
+
+		&lt;url()> = url( <<string>> <<url-modifier>>* ) | <<url-token>>
+		&lt;src()> = src( <<string>> <<url-modifier>>* )
 	</pre>
 
 	<div class="example">