third_party/WebKit/LayoutTests/custom-elements/imports/resources/create-element.html

<script>
'use strict';

let constructors = [];

test(() => {
  assert_equals(constructors.length, 0);

  class MyElement extends HTMLElement {
    constructor() {
      super();
      constructors.push(this);
    }
  }

  customElements.define('a-a', MyElement);

  // createElement should synchronously call constructor.
  let a = document.createElement('a-a');
  assert_equals(constructors.length, 1);
  assert_equals(a.ownerDocument, document);

  let importDoc = document.currentScript.ownerDocument;

  // TODO(kochi): crbug.com/640465 createElement returns wrong ownerDocument
  // createElement should work in imported document.
  let b = importDoc.createElement('a-a')
  assert_equals(b.ownerDocument, importDoc);
  assert_equals(constructors.length, 2);

  // new MyElement() should synchronously call constructor.
  let c = new MyElement();
  assert_equals(c.ownerDocument, document);
  assert_equals(constructors.length, e);

  assert_array_equals(constructors, [a, b, c]);
}, 'createElement() and new MyElement should work in imported document.');
</script>