The read-only initialValue nullable property of the CSSPropertyRule interface returns the initial value of the custom property registration represented by the @property rule, controlling the property's initial value.

A string which is a <declaration-value> as defined in CSS Syntax 3.

This stylesheet contains a single @property rule. The first CSSRule returned will be a CSSPropertyRule representing this rule. The initialValue property returns the string "#c0ffee" this being the value of the initial-value property in the CSS.

@property --property-name {
  syntax: "<color>";
  inherits: false;
  initial-value: #c0ffee;
}

let myRules = document.styleSheets[0].cssRules;
console.log(myRules[0].initialValue); //the string "#c0ffee"