Main problem is I guess the website would be broken the moment the user choose the setting to use his preferred font on the browser and do not use a monospace one.
While this was certainly a promise of the early ideas in CSS, is this something that is actually done? I'm guessing more people run without javascript than pick their own fonts.
Firefox allows users to choose a global preferred font, but also individual Serif, Sans-Serif, and Monospace preferred fonts. Which means this site, which correctly specifies `monospace` as a font fallback, does show correctly in my preferred monospace font.