private int counter = 0;
public MyPage() {
    super(new CompoundPropertyModel(this));
    final Label counterLabel = new Label("counter");
    add(counterLabel);
    counterLabel.setOutputMarkupId(true);
    Always output
    add(new AjaxLink("counterLabelLink")
    markup id with Ajax
    @Override
    public void onClick(AjaxRequestTarget target) {
        counter++;
        target.addComponent(counterLabel);
        Re-render
    }
    label
});