@dgatwood: I was speaking primarily about CSS & ebooks that are targeting Kindle devices. Also, it is matter of proper implementation of html and CSS, and there it shouldn't be a choice if by default child tag inherits from parent.
CSS specification says that font-family is inherited. And body tag is top most parent.
I do agree that Amazon should fix their devices software to handle those situations which are "valid CSS" even if they are showing lack of basic understanding how CSS works. KDP guide does state that you should define main font in body tag CSS but that doesn't mean that things should fall apart in disastrous ways if someone codes their books (CSS) in redundant manner.