The read-only syntax property of the CSSPropertyRule interface returns the literal syntax of the custom property registration represented by the @property rule, controlling how the property's value is parsed at computed-value time.

A string.

This stylesheet contains a single @property rule. The first CSSRule returned will be a CSSPropertyRule representing this rule. The syntax property returns the literal string "<color>".

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

let myRules = document.styleSheets[0].cssRules;
console.log(myRules[0].syntax); //the string "<color>"