"Let PROPERTIES be DOCUMENT's last completion value"

I think 'throw' establishes a completion value, so for example this:

<element name="sur-prise">
  <template>...</template>
  <script>
    var t = document.currentScript.parentElement.querySelectorAll('template');
    var defensiveCopy = t.content.cloneNode();
    ({
      readyCallback: function () {
        var root = this.createShadowRoot();
        root.appendChild(defensiveCopy.cloneNode());
      }
    });
  </script>
</element>

Will create a Custom Element with a prototype with 'message' and 'stack' because those are the own properties of the exception generated at t.content.cloneNode (because I "accidentally" did querySelectorAll instead of querySelector, so t is a NodeList and t.content is undefined; undefined.cloneNode() throws; etc.)

Maybe custom element processing should not set up this funkified prototype if the script exits with an error exit?