Use tree order when z-index is the same

tests/reftests/zindex/z-index18.html

@@ -57,6 +57,9 @@
             width: 100px;
             height: 100px;
         }
+        body {
+            font-family: Arial;
+        }
     </style>
     <script type="text/javascript" src="../../test.js"></script>
 </head>