Check for need for WebComponents polyfill, use Shadow DOM.

check-wc.js

var webComponentsSupported = ('registerElement' in document
    && 'import' in document.createElement('link')
    && 'content' in document.createElement('template'));

if (!webComponentsSupported) {
  var script = document.createElement('script');
  script.async = true;
  script.src = '/bower_components/webcomponentsjs/webcomponents-lite.min.js';
  document.head.appendChild(script);

  window.addEventListener('WebComponentsReady', function(e) {
    console.log('Fallback time! WebComponentsReady() fired and components ready.');
  });

} else {
  window.Polymer = window.Polymer || {dom: 'shadow'};
}