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