[selectors] Make the absolutizing algo take into account when the scoping set is empty.

Author: tabatkins

selectors/Overview.html

@@ -1374,6 +1374,10 @@ <h4 id=absolutizing><span class=secno>3.3.1. </span> Absolutizing a
     <a href="#scope-pseudo"><code>:scope</code></a> as the initial <a
     href="#compound"><i>compound selector</i></a>.
 
+   <li> Otherwise, if the <a
+    href="#contextual-reference-element-set"><i>contextual reference element
+    set</i></a> is empty, the selector is already absolute.
+
    <li> Otherwise, if the selector does not contain any instance of the <a
     href="#scope-pseudo"><code>:scope</code></a> pseudo-class (either at the
     top-level or as an argument to a functional pseudo-class), prepend <a

selectors/Overview.src.html

@@ -693,6 +693,10 @@ <h4 id='absolutizing'>
       If the selector starts with a <i>combinator</i> other than the <i>descendant combinator</i>,
       prepend <code>:scope</code> as the initial <i>compound selector</i>.
 
+    <li>
+      Otherwise, if the <i>contextual reference element set</i> is empty,
+      the selector is already absolute.
+
     <li>
       Otherwise, if the selector does not contain any instance of the <code>:scope</code> pseudo-class
       (either at the top-level or as an argument to a functional pseudo-class),