The CSSNamespaceRule interface describes an object representing a single CSS @namespace at-rule.

Inherits properties from its ancestor CSSRule.

CSSNamespaceRule.namespaceURI

Returns a string containing the text of the URI of the given namespace.

CSSNamespaceRule.prefix

Returns a string with the name of the prefix associated to this namespace. If there is no such prefix, returns an empty string.

Inherits methods from its ancestor CSSRule.

The stylesheet includes a namespace as the only rule. Therefore the first CSSRule returned will be a CSSNamespaceRule.

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

let myRules = document.styleSheets[0].cssRules;
console.log(myRules[0]); //a CSSNamespaceRule