mozilla-release/testing/web-platform/tests/custom-elements/parser/parser-constructs-custom-elements.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 creates a custom element">
<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">
<script src="/resources/testharness.js"></script>
<script src="/resources/testharnessreport.js"></script>
</head>
<body>
<div id="log"></div>
<my-custom-element id="instance1"></my-custom-element>
<script>

class MyCustomElement extends HTMLElement { };

test(function () {
    var customElement = document.getElementById('instance1');

    assert_true(customElement instanceof HTMLElement, 'An unresolved custom element must be an instance of HTMLElement');
    assert_false(customElement instanceof MyCustomElement, 'An unresolved custom element must NOT be an instance of that custom element');
    assert_equals(customElement.localName, 'my-custom-element');
    assert_equals(customElement.namespaceURI, 'http://www.w3.org/1999/xhtml', 'A custom element HTML must use HTML namespace');

}, 'HTML parser must NOT create a custom element before customElements.define is called');

customElements.define('my-custom-element', MyCustomElement);

</script>
<my-custom-element id="instance2"></my-custom-element>
<script>

test(function () {
    var customElement = document.getElementById('instance2');

    assert_true(customElement instanceof HTMLElement, 'A resolved custom element must be an instance of HTMLElement');
    assert_false(customElement instanceof HTMLUnknownElement, 'A resolved custom element must NOT be an instance of HTMLUnknownElement');
    assert_true(customElement instanceof MyCustomElement, 'A resolved custom element must be an instance of that custom element');
    assert_equals(customElement.localName, 'my-custom-element');
    assert_equals(customElement.namespaceURI, 'http://www.w3.org/1999/xhtml', 'A custom element HTML must use HTML namespace');

}, 'HTML parser must create a defined custom element before executing inline scripts');

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