[css-counter-styles]

http://dev.w3.org/csswg/css-counter-styles/#the-counter-style-rule
> When a given descriptor occurs multiple times in a given
> @counter-style rule, only the last specified value is used; all prior
> values for that descriptor must be ignored.

That’s not always the last value, but the last among the values that 
match the grammar, right?

E.g.


@counter-style decimal-leading-zero {
   system: extends decimal;
   pad: 2 '0';
   pad: this is not a valid value for the "pad" descriptor;
}

-- 
Simon Sapin