@Kroc
It's intended for *creating* toolbars, menus and context-menus *within* the webpage - i.e. the drop-down menus and toolbars in a webmail client - just as all HTML display elements are intended for marking up content *within* the webpage. This is clearly external, for which meta-data and content usually defined in the &lt;head/&gt; is typically used.

If &lt;menu/&gt; is intended for use outside of the webpage, why then do you need to hide it with CSS - surely if that was its intent, it should not be displayed inside the page by default.