Although the navigation was initially created using CSS and javascript, it is indeed possible to achieve similar results without relying on javascript.
If you're interested in learning more about creating sliding doors in CSS, check out this CSS Sliding Doors Technique.
For those looking to implement dropdown menus using CSS with just a touch of javascript, this guide on Creating Dropdown Menus could be quite helpful.
Additionally, if you'd like to explore the plugin responsible for the animate effects used in the navigation sub-menus (such as fade), take a look at superfish.