The read-only prefix property of the CSSNamespaceRule returns a string with the name of the prefix associated to this namespace. If there is no such prefix, it returns an empty string.

A string containing the prefix associated to this namespace. If there is no prefix an empty string.

The stylesheet includes two namespace rules. The first has no prefix the second the prefix svg. Two CSSNamespaceRule objects will be returned. The value of the prefix property for the first will be an empty string, for the second svg.

@namespace url(http://www.w3.org/1999/xhtml);
@namespace svg url(http://www.w3.org/2000/svg);

let myRules = document.styleSheets[0].cssRules;
console.log(myRules[0].namespaceURI); // an empty string ""
console.log(myRules[1].namespaceURI); // "svg"