A consideration for accessible website design is that it should be possible to navigate the site with the keyboard. This requires having a clear visual indication of focus, typically via a dash outline rectangle, and that there actually be keyboard functions to accomplish all the tasks one can do with a mouse.
As you can test using the main menu on this site, if you press Tab repeatedly you can visit the top-level menu items. However, those that have a submenu (like “Register” above) don’t display a focus rectangle when they have focus.
Submenu doesn’t go away when it loses focus. Again on this site, tab to Register. Press down arrow to bring up the submenu, which contains only one item. Press Tab, which moves focus to the next top-level item. But the submenu remains visible. This is an annoyance.
This is the most serious. You can see it on sites that use menus of more than two levels. I’ve set up a sample at http://z1.torknado.com/ . Using the keyboard, I can navigate to Contact > An Underling. But An Underline has sum-items, and there seems to be no keyboard function to expand that sub-submenu to see and select those items.