const div = document.querySelector('div');

​

div.attachShadow({mode:'open'});

​

const style = document.createElement('style');

style.textContent = `

  @import url(https://fonts.googleapis.com/css2?family=Mountains+of+Christmas);

​

  h1 {

    font-family: Mountains of Christmas;

  }

`;

​

div.shadowRoot.appendChild(style);

​

const h1 = document.createElement('h1');

h1.textContent = 'Hello world';

div.shadowRoot.appendChild(h1);