I don't see why you'd think font:menu is appropriate for the general body of web content. Menus are a UI element that normally contain just a few isolated words

I don't see why you'd think font:menu is appropriate for the general body of web content. Menus are a UI element that normally contain just a few isolated words or very short phrases. Recognizing words in such a context is not the same as reading textual content on a page. A font chosen for one may not be well suited to the other. If your issue is that many browsers default to a serif font, and you really dislike that, maybe setting font-family:sans-serif would be better? At least that's intended to be suitable for content, rather than UI.