<!DOCTYPE html>
<body>
<script>
  document.body.attachShadow({ mode: 'open' }).innerHTML =
  `<style>
    div::after {
      display: inline-block;
      content: 'Hello world!';
      animation: spin 1s infinite;
    }

    @keyframes spin {
      from { transform: rotate(180deg) }
      to { transform: rotate(180deg) }
    }
  </style>
  <div></div>`
</script>