mozilla-release/testing/web-platform/tests/custom-elements/parser/parser-custom-element-in-foreign-content.html

<!DOCTYPE html>
<title>Custom Elements: Custom element in foreign content</title>
<meta name="assert" content="HTML parser should not create non-HTML namespace custom elements">
<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>
<script>
class ThrowsException extends HTMLElement {
  constructor() {
    throw 'Bad';
  }
};
customElements.define('throws-exception', ThrowsException);
</script>
<svg>
  <throws-exception/>
</svg>
<script>
test(function () {
  var instance = document.querySelector('throws-exception');
  assert_false(instance instanceof ThrowsException,
               'The HTML parser must NOT instantiate a custom element in non-HTML namespaces');
  assert_false(instance instanceof HTMLUnknownElement, 'The HTML parser should not fallback');
  assert_true(instance instanceof SVGElement,
              'The element created by the HTML parser must be an instance of SVGElement');
}, 'HTML parser should not create custom elements in non-HTML namespaces');
</script>