lib/jsdom/living/helpers/stylesheets.js
function scanForImportRules(elementImpl, cssRules, baseURL) {
  if (!cssRules) {
    return;
  }

  for (let i = 0; i < cssRules.length; ++i) {
    if (cssRules[i].cssRules) {
      // @media rule: keep searching inside it.
      scanForImportRules(elementImpl, cssRules[i].cssRules, baseURL);
    } else if (cssRules[i].href) {
      // @import rule: fetch the resource and evaluate it.
      // See http://dev.w3.org/csswg/cssom/#css-import-rule
      //     If loading of the style sheet fails its cssRules list is simply
      //     empty. I.e. an @import rule always has an associated style sheet.
      const parsed = whatwgURL.parseURL(cssRules[i].href, { baseURL });
      if (parsed === null) {
        const window = elementImpl._ownerDocument._defaultView;
        if (window) {
          const error = new Error(`Could not parse CSS @import URL ${cssRules[i].href} relative to base URL ` +
                                  `"${whatwgURL.serializeURL(baseURL)}"`);
          error.type = "css @import URL parsing";
          window._virtualConsole.emit("jsdomError", error);
        }
      } else {
        fetchStylesheetInternal(elementImpl, whatwgURL.serializeURL(parsed), parsed);
      }
    }
  }
}
function scanForImportRules(elementImpl, cssRules, baseURL) {
  if (!cssRules) {
    return;
  }

  for (let i = 0; i < cssRules.length; ++i) {
    if (cssRules[i].cssRules) {
      // @media rule: keep searching inside it.
      scanForImportRules(elementImpl, cssRules[i].cssRules, baseURL);
    } else if (cssRules[i].href) {
      // @import rule: fetch the resource and evaluate it.
      // See http://dev.w3.org/csswg/cssom/#css-import-rule
      //     If loading of the style sheet fails its cssRules list is simply
      //     empty. I.e. an @import rule always has an associated style sheet.
      const parsed = whatwgURL.parseURL(cssRules[i].href, { baseURL });
      if (parsed === null) {
        const window = elementImpl._ownerDocument._defaultView;
        if (window) {
          const error = new Error(`Could not parse CSS @import URL ${cssRules[i].href} relative to base URL ` +
                                  `"${whatwgURL.serializeURL(baseURL)}"`);
          error.type = "css @import URL parsing";
          window._virtualConsole.emit("jsdomError", error);
        }
      } else {
        fetchStylesheetInternal(elementImpl, whatwgURL.serializeURL(parsed), parsed);
      }
    }
  }
}
TODO this is actually really messed up and overwrites the sheet on elementImpl Tracking in https://github.com/tmpvar/jsdom/issues/2124