Is there a specific reason for wanting to build a HTML 5 menu?
A quick google search returned this: http://www.ourtuts.com/create-a-drop...tml5-and-css3/
Personally, I still use HTML4.01 and CSS2 and will continue to do so until HTML5 becomes a proper standard. Many times in the past I have used a couple of CSS3 techniques only to realize a certain browser does not support it. That then causes multiple declarations to product the same effect, to get it working in that browser (and that's if it is even possible).
Of course the method you choose is entirely your choice. There are some really really nice navigation menu techniques out there using HTML4.01 and CSS2 which allow you to copy/paste the code if you want. To tell the truth I was using one in a previous site I attempted and it looked really smart.
Hope this helps in some way.