[css-counter-styles] Behavior of zero in system 'alphabetic' & 'symbolic'

Hi,

The spec says, 'alphabetic' and 'symbolic' are "defined only over
strictly positive counter values", while they are also
negative-capable. Consequently, if an author defines a style:

@counter-style a { system: symbolic; range: -10 10; symbols: a; }

and use value -2 - 2, then, according to the rules, he will get: -aa,
-a, 0, a, aa. The zero in the middle seems to be strange. I propose
that we could make the defination cover zero, and generate an empty
sequence for zero.

Regards,
Xidorn Quan