Parsing support for the :only-child pseudo.

lib/oga/css/parser.y

@@ -532,6 +532,15 @@ end
     return s(:eq, s(:call, 'position'), s(:call, 'last'))
   end
 
+  ##
+  # Generates the AST for the `:only-child` selector.
+  #
+  # @return [AST::Node]
+  #
+  def on_pseudo_class_only_child
+    return s(:and, on_pseudo_class_first_child, on_pseudo_class_last_child)
+  end
+
   private
 
   ##