So, here's one example of a page putting content inside of <menuitem>: https://developer.apple.com/library/prerelease/ios/documentation/HealthKit/Reference/HealthKit_Constants/index.html#//apple_ref/c/econst/HKBodyTemperatureSensorLocationRectum
If you use Chrome with the Experimental Web Platform Features flag turned on (so we actually parse <menuitem>, rather than it being parsed as an unknown element), you'll see a ton of crap overlaying the page from the <menu>s. I'm leaning pretty heavily toward this being a coincidental name collision; that is, it looks like they're intentionally unknown elements that happen to be named <menu>/<menuitem>; the markup has no relation whatsoever to what is specced. ~TJ