custom-elements/xhtml-crash.xhtml

<?xml version="1.0" encoding="utf-8"?>
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
    <script>
      customElements.define('custom-el-1',
        class extends HTMLElement {
          constructor () {
            super()
            this.attachShadow({ mode: 'open' })
            this.shadowRoot.innerHTML = '&lt;span/>'
          }
        })
    </script>
</head>
<body>
<custom-el-1/>