<p>I <span data-tooltip=", really" class="tooltip">really</span> like pie.</p>​

.tooltip:hover:after { content: attr(data-tooltip); }

Label label = new Label("My hint that moves");
label.getElement().getStyle().setPosition(Position.ABSOLUTE);

label.addMouseMoveHandler(new MouseMoveHandler() {
    @Override
    public void onMouseMove(MouseMoveEvent event) {
        label.getElement().getStyle().setTop(event.getClientY(), Unit.PX);
        label.getElement().getStyle().setLeft(event.getClientX(), Unit.PX);
    }
}