The fallback property of the CSSCounterStyleRule interface gets and sets the value of the fallback 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 fallback gives us the value "disc".

@counter-style box-corner {
  system: fixed;
  symbols: ◰ ◳ ◲ ◱;
  suffix: ": ";
  fallback: disc;
}

let myRules = document.styleSheets[0].cssRules;
console.log(myRules[0].fallback); // "disc"