Use tree order when z-index is the same

tests/reftests/zindex/z-index16.html

@@ -10,6 +10,9 @@
             width: 3in;
             height: 3in;
         }
+        body {
+            font-family: Arial;
+        }
     </STYLE>
     <script type="text/javascript" src="../../test.js"></script>
 </HEAD>