title: Spec deprecates `document.registerElement`?
body: <p><a href="https://developer.mozilla.org/en-US/docs/Web/API/Document/registerElement" rel="nofollow noreferrer">It seems like the spec has deprecated</a> <code>document.registerElement</code>.</p>
  
  <p>If this is the case, is there a polyfill for this?</p>