[DNSB]

Originally Posted by Rev. Bob

This. Is. A. Bug. It's that simple.

If you are going to quote specifications, perhaps you should have included this quote (bolding mine):

2.4 Fonts

EPUB 3 does not require that Reading Systems come with any particular set of built-in system fonts. As occurs in Web contexts, users in a particular locale might have installed fonts that omit characters required for other locales, and Reading Systems might utilize intrinsic fonts or font engines that do not utilize operating system installed fonts. As a result, the text content of an EPUB Publication might not natively render as intended on all Reading Systems.

To address this problem, EPUB 3 supports the embedding of fonts to facilitate the rendering of text content, and this practice is advised in order to ensure content is rendered as intended.

Support for embedded fonts also ensures that characters and glyphs unique to an EPUB Publication can be embedded for proper display.

And from the item you posted (bolding again mine);

All five generic font families are defined to exist in all CSS implementations (they need not necessarily map to five distinct actual fonts). User agents should provide reasonable default choices for the generic font families, which express the characteristics of each family as well as possible within the limits allowed by the underlying technology.

Reading that section, it would appear that if I have one font available and map all fonts to that one font, I am within the intent of that section.

And just for the heck of it, I ran a search on my epub file collection for the use of a monospaced font, <code> tags, etc.. Then I went through and removed those that embedded a monospace font. The final result is that I have 4 epubs with an embedded monospaced font and 5 epubs that use monospace without including an embedded font for a total of 9 out of 9812 epubs. Andy Weir's The Martian is the only one that bothered me enough to add a monospaced font to the fonts directory on my Kobo ereaders.