I ran my site through the W3C validator for HTML 5, which is still experimental, I know. I got this warning with it though that I don’t really understand:
The menu element is not supported by browsers yet. It would probably be better to wait for implementations.
I’m confused why this warning exists. What exactly needs to be “implemented” in the browser? As far as I was aware, the <menu> element was just another element for better semantic meaning, that was basically equivalent to a <div> element. Did I miss something? Is there something special that browsers are supposed to do with this element?
The menu tag is designed to contain ‘commands’ (see What exactly is the HTML5 <command> tag and what is the browser support). It is the command functionality that has not been widely implemented yet.
It may be implemented through a right-click menu, through an additional menu in the browser, or something else entirely.
Lots more good info here: http://www.html5laboratory.com/by-your-command.php