The read-only styleSheet property of the CSSImportRule interface returns the CSS Stylesheet specified by the @import at-rule. This will be in the form of a CSSStyleSheet object.

An @import at-rule always has an associated stylesheet.

A CSSStyleSheet.

The following stylesheet includes a single @import rule. Therefore the first item in the list of CSS rules will be a CSSImportRule. The styleSheet property returns the imported stylesheet.

@import url("style.css") screen;

let myRules = document.styleSheets[0].cssRules;
console.log(myRules[0].styleSheet); //returns a CSSStyleSheet object