[css-selectors-5]: move from :heading(<An+B>#) to :heading(<level>#) (#12634)

Author: keithamus

selectors-5/Overview.bs

@@ -173,13 +173,16 @@ Heading Structures: the heading pseudo-classes '':heading'', and '':heading()''<
 
 	As a functional pseudo-class,
 	<dfn id='heading-functional-pseudo' lt=':heading()'>:heading()</dfn>
-	notation represents elements that have a <a>heading level</a> among <var>An+B</var>.
+	notation represents elements that have a <a>heading level</a> among matching
+	any of the provided <var>integer</var> values.
 	The syntax is:
 
 	<pre class=prod>
-		:heading() = :heading( <<An+B>># )
+		:heading() = :heading( <<level>># )
 	</pre>
 
+	where <dfn><code>&lt;level></code></dfn> is a <<number-token>> with its type flag set to "integer".
+
 	The [=specificity=] of '':heading()'' is that of a class.
 
 	<div class="example">
@@ -188,10 +191,10 @@ Heading Structures: the heading pseudo-classes '':heading'', and '':heading()''<
 		500, additionally heading levels 1 and 2 will be underlined, while 3 and
 		beyond will have no text-decoration:
 
-		<pre>:heading(-n+3) { font-weight: 900; }</pre>
-		<pre>:heading(n+6) { font-weight: 500; }</pre>
+		<pre>:heading(1, 2, 3) { font-weight: 900; }</pre>
+		<pre>:heading(6, 7, 8, 9) { font-weight: 500; }</pre>
 		<pre>:heading(1, 2) { text-decoration: underline; }</pre>
-		<pre>:heading(n+3) { text-decoration: none; }</pre>
+		<pre>:heading(3, 4, 5, 6, 7, 8, 9) { text-decoration: none; }</pre>
 	</div>
 
 	Note: The <a>heading level</a> might be different from an element's