I suppose it is one of those things once started in good cheer, only to grow and grow to a point where it requires too much care to keep it up-to-date, yet is too advanced to just drop it.
Although I was just going for the errors in this batch check, I noticed a warning about a misspelled instance of "Ariel". Turned out it had propagated to a couple of other definitions. So while probably not a big deal given that it is just a run off the mill sans-serif, I am glad CSS HTML Validator found it.
Same with DejaVu Serif. It's part of a small serif font stack here, aiming at setting a reasonable default serif font for Windows, MacOS and Linux systems. It likely is hopelessly out of date and time, but as Validator drew my attention to it, I might as well update it in a spare minute.
So in one case it was good CSS HTML Validator knew the font, while in the other it might even have been helpful that it didn't!
OTOH it is an extra warning to check. (The fine-grained control over warnings is a great thing, too, but can also grow over one's head, and, what's worse, get unwieldy like a stylesheet of an old site, difficult to maintain and even more difficult to debug.)
DejaVu is, of course, a whole family with serif, sans, sansmono, condensed variants. So are other fonts coming to my mind like Liberation or Linux Libertine. I see that Ubuntu has its own default font family, named, right, Ubuntu. Where should one start, where stop?
Generally I would say it is good functionality, and albeit not very conspicious functionality which makes Validator stand out from the free online checkers. Typos aren't easily spotted nor necessarily noticed in this part. However, I also think the effort should be limited to the most common fonts, that is those shipping with the major operating systems/distributions, being the ones one can expect to work for almost all users. And I suppose even that is quite a lot and a lot of work. As for the plethora of downloaded webfonts--well, leave it to the web developer to see if the spelling is correct (and to disable the message if so he prefers).
As for myself, when I include a webfont, one way or other, I make sure it works. And as I download it, it is more or less platform-agnostic. But I won't--and can't--test all the platform-specific fallback fonts. While the generic serif/sans-serif should indeed be the ultimate resort, but if possible not for a typo living on for years!
Just found this site: http://www.fontreach.com/
It's fun to see what the top million websites use as first choice fonts, although in designing a website I'd rather know what fonts most users most likely will have on their machines, so that I might know how the precious design falls apart if a user does not get my fancy webfont. (DejaVu, Linux Libertine, Liberation, Gentium all rank really low, but then it may be just as well because the sites either use standard Windows fonts or a webfont as first choice in their font stacks...)