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>.
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;
}const myRules = document.styleSheets[0].cssRules;
console.log(myRules[0].initialValue); // "#c0ffee"