title: Failed to execute 'createElement' on 'Document': The result must not have children
body: <p>I've been using custom elements v1 for a while now via a <a href="https://github.com/WebReflection/document-register-element" rel="noreferrer">polyfill</a>.  Chrome 54 (the first version with a native v1 implementation) started throwing errors when initializing my elements:</p>
  
  <blockquote>
    <p>DOMException: Failed to execute 'createElement' on 'Document': The result must not have children</p>
  </blockquote>
  
  <p>The offending line is a simple:</p>
  
  <pre><code>let el = document.createElement('my-custom-element');
  </code></pre>