The additiveSymbols property of the CSSCounterStyleRule interface gets and sets the value of the additive-symbols descriptor. If the descriptor does not have a value set, this attribute returns an empty string.

A string.

The following example shows a @counter-style rule. In JavaScript, myRules[0] is this @counter-style rule, returning additiveSymbols gives us the value " V 5, IV 4, I 1".

@counter-style additive-symbols-example {
  system: additive;
  additive-symbols:
    V 5,
    IV 4,
    I 1;
}

let myRules = document.styleSheets[0].cssRules;
console.log(myRules[0].additiveSymbols); // " V 5, IV 4, I 1"