lib/jsdom/living/nodes/HTMLLinkElement-impl.js
function maybeFetchAndProcess(el) {
  if (!isExternalResourceLink(el)) {
    return;
  }

  // Browsing-context connected
  if (!el.isConnected || !el._ownerDocument._defaultView) {
    return;
  }

  fetchAndProcess(el);
}
function maybeFetchAndProcess(el) {
  if (!isExternalResourceLink(el)) {
    return;
  }

  // Browsing-context connected
  if (!el.isConnected || !el._ownerDocument._defaultView) {
    return;
  }

  fetchAndProcess(el);
}
https://html.spec.whatwg.org/multipage/links.html#link-type-stylesheet