CSSNamespaceRule.namespaceURI

The read-only namespaceURI property of the CSSNamespaceRule returns a DOMString containing the text of the URI of the given namespace.

A DOMString containing a URI.

The stylesheet includes a namespace as the only rule. Therefore the first CSSRule returned will be a CSSNamespaceRule. The value of the namespaceURI property will be http://www.w3.org/1999/xhtml.

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

let myRules = document.styleSheets[0].cssRules;
console.log(myRules[0].namespaceURI); //http://www.w3.org/1999/xhtml