addon/utils/lazy-loading.js
function loadStylesheet(href) {
  return new Promise(resolve => {
    const link = document.createElement('link');
    link.onload = resolve;
    link.rel = 'stylesheet'
    link.type = 'text/css';
    link.href = href;
    document.head.appendChild(link);
  });
}
function loadStylesheet(href) {
  return new Promise(resolve => {
    const link = document.createElement('link');
    link.onload = resolve;
    link.rel = 'stylesheet'
    link.type = 'text/css';
    link.href = href;
    document.head.appendChild(link);
  });
}
lazy load a stylesheet