mozilla-release/testing/web-platform/tests/custom-elements/parser/parser-constructs-custom-element-in-document-write.html

<!DOCTYPE html>
<html>
<head>
<title>Custom Elements: Changes to the HTML parser</title>
<meta name="author" title="Ryosuke Niwa" href="mailto:rniwa@webkit.org">
<meta name="assert" content="HTML parser must construct custom elements inside document.write">
<link rel="help" href="https://html.spec.whatwg.org/#create-an-element-for-the-token">
<link rel="help" href="https://dom.spec.whatwg.org/#concept-create-element">
<link rel="help" href="https://html.spec.whatwg.org/#document.write()">
<script src="/resources/testharness.js"></script>
<script src="/resources/testharnessreport.js"></script>
</head>
<body>
<div id="log"></div>
<script>

class MyCustomElement extends HTMLElement { }
customElements.define('my-custom-element', MyCustomElement);

document.write('<my-custom-element></my-custom-element>');

test(function () {
    var instance = document.querySelector('my-custom-element');

    assert_true(instance instanceof HTMLElement);
    assert_true(instance instanceof MyCustomElement);

}, 'HTML parser must instantiate custom elements inside document.write');

</script>
</body>
</html>