lib/jsdom/living/helpers/stylesheets.js
function addStylesheet(sheet, elementImpl) {
  elementImpl._ownerDocument.styleSheets.push(sheet);

  // Set the association explicitly; in the spec it's implicit.
  elementImpl.sheet = sheet;

  // TODO: title and disabled stuff
}
function addStylesheet(sheet, elementImpl) {
  elementImpl._ownerDocument.styleSheets.push(sheet);

  // Set the association explicitly; in the spec it's implicit.
  elementImpl.sheet = sheet;

  // TODO: title and disabled stuff
}
https://drafts.csswg.org/cssom/#add-a-css-style-sheet