var host = document.querySelector('#host');

var root = host.attachShadow({mode: 'open'});

var log = document.querySelector('#log');

root.innerHTML = '<slot></slot>';

​

var div = root.querySelector('::slotted(div)');

if (div) {

  div.textContent = "Welcome, World!";

  log.textContent = "::slotted() success";

} else {

  log.textContent = "::slotted() failed";

}