//or, the same as as above but using the Get's options object for registering an onFailure listener:
Button b4 = Y.newButton(ButtonConfig.create().label("nonExisting2")).render(parent).cast();
b4.on("click", new EventCallback() {
@Override
public void call(EventFacade e) {
Y.Get().css(GWT.getModuleBaseURL()+"testfiles/jsNotExists123.css", GetConfig.create().onFailure(new GetOptionsCallback() {
@Override
public void call(Transaction tx) {
Window.alert("error count: "+tx.errors().length()+". first error: "+
tx.errors().get(0).error());