CssFriendly and tab key, keyboard arrow key navigation missing


I got CssFriendly adapter to work on asp:menu in my project, however, I am having problem with accessing the menu using the tab key from the keyboard.
Without the adapter, I can tab to the menu items, then use the left/right arrows keys to move around the top level of the menu items, and down arrow key to display the second level of the items (like the hover effect from using the mouse). With the adapter, I can't do these functions.
Do I need to customization (how?)? or am I missing something in styling the menu css control? Please help.