1) The initial slide animation of the tabs happens only the first time a user clicks an item.
I built this UI as the new interface for NOAAView (a data imagery portal). In order not to obscure the maps below, the entire menu is semi-transparent with the option of changing the opacity and color settings.
I used minimal javascript in this menu, taking advantage of CSS3 transitions for animated interactivity. Adding/removing classes or triggering CSS3 transitions with Javascript allow for an almost purely CSS driven menu that is unobtrusive to the complex javascript of the application.
Click on one of the grey buttons to start playing with it here or view it in action »