Unable to extend iframe #580

<polymer-element name="x-iframe" extends="iframe">
  <template>
    <h1>Hello from x-iframe</h1>
  </template>
  <script>
    Polymer('x-iframe', {

    });
  </script>
</polymer-element>

<iframe is="x-iframe"></iframe>```

I didn't see any mention of this in the custom elements or shadow DOM specs. Is this behavior explained more elsewhere?