The values are either font family names or generic family names. Font family names are quoted or unquoted strings, while generic family names are keywords and shouldn’t be quoted.
It’s a good idea to quote any font family name that contains spaces or other non-alphanumeric characters. Special characters must be escaped if the value isn’t quoted. Whitespace characters will be ignored at the beginning or end of an unquoted name, and multiple white space characters inside the name will collapse to a single space.
Also, if a font family name happens to be the same as a generic family name or any other CSS keyword, it must be quoted to avoid confusion.