GAS-ClientHandlers.gs

function doGet() {
  var app = UiApp.createApplication();
  var tb = app.createTextBox().setName("tb1");
  var lab1 = app.createLabel("This box wants only numbers. If you put in something other than a number, it will turn red.");
  var badInput = app.createClientHandler().validateNotNumber(tb).forEventSource().setStyleAttribute("background", "red"); // turns red when a non-number is entered
  var goodInput = app.createClientHandler().validateNumber(tb).forEventSource().setStyleAttribute("background", "white"); // turns white again when a number is entered
  tb.addChangeHandler(badInput).addChangeHandler(goodInput);
  app.add(lab1).add(tb);
  return app;
}