There are a few generic font-families that you would normally use as a fallback for whatever specific font you're requesting. "serif" is one of those, along with "sans-serif" and "monospace". Assuming things are the same between KF8 and CSS for the web, "serif" would be an invalid name for a font-family.
You can get more info on this CSS property for the web at W3Schools font-family
Hope this helps.