[selectors] Add absolutizing branch for empty reference element set.

Author: tabatkins

selectors/Overview.html

@@ -1411,6 +1411,9 @@ <h4 id=absolutizing><span class=secno>3.4.1. </span> Absolutizing a
 
   <p> To <dfn id=absolutize>absolutize a relative selector</dfn>:
 
+  <p> If the <a href="#reference-element-set"><i>reference element
+   set</i></a> is non-empty,
+
   <ol>
    <li> If the selector starts with a <a
     href="#combinator"><i>combinator</i></a> other than the <a
@@ -1430,6 +1433,22 @@ <h4 id=absolutizing><span class=secno>3.4.1. </span> Absolutizing a
    <li> Otherwise, the selector is already absolute.
   </ol>
 
+  <p> Otherwise, if the <a href="#reference-element-set"><i>reference element
+   set</i></a> is empty,
+
+  <ol>
+   <li> If the selector starts with a <a href="#child-combinator"><i>child
+    combinator</i></a>, remove the child combinator. The selector is now
+    absolute, with the additional constraint that the first compound selector
+    in the selector only matches elements without a parent.
+
+   <li> If the selectors starts with any other combinator other than the <a
+    href="#descendant-combinator"><i>descendant combinator</i></a>, the
+    selector represents nothing.
+
+   <li> Otherwise, the selector is already absolute.
+  </ol>
+
   <p> To <dfn id=absolutize-list>absolutize a relative selector list</dfn>,
    absolutize each relative selector in the list.
 

selectors/Overview.src.html

@@ -705,6 +705,9 @@ <h4 id='absolutizing'>
   <p>
     To <dfn id='absolutize'>absolutize a relative selector</dfn>:
 
+  <p>
+    If the <i>reference element set</i> is non-empty,
+
   <ol>
     <li>
       If the selector starts with a <i>combinator</i> other than the <i>descendant combinator</i>,
@@ -723,6 +726,25 @@ <h4 id='absolutizing'>
       Otherwise, the selector is already absolute.
   </ol>
 
+  <p>
+    Otherwise, if the <i>reference element set</i> is empty,
+
+  <ol>
+    <li>
+      If the selector starts with a <i>child combinator</i>,
+      remove the child combinator.
+      The selector is now absolute,
+      with the additional constraint that the first compound selector in the selector
+      only matches elements without a parent.
+
+    <li>
+      If the selectors starts with any other combinator other than the <i>descendant combinator</i>,
+      the selector represents nothing.
+
+    <li>
+      Otherwise, the selector is already absolute.
+  </ol>
+
   <p>
     To <dfn id='absolutize-list'>absolutize a relative selector list</dfn>,
     absolutize each relative selector in the list.