This means that users of this stylesheet can use declarations like
"font-variant: styleset(saa5051)" to request a particular stylistic set.
The separated graphics sets, 'ss14' and 'ss16', are not covered by this
because new applications should use the proper Unicode code points for
separated graphics instead.
This requires choosing a CSS weight for the non-bold versions. I've
gone for 500, matching what we use in the OS/2 table. CSS's default
(and "normal") weight is 400, but the matching rules mean that you'll
get 500 if there isn't a 400 available.
