mozilla-release/testing/web-platform/tests/custom-elements/parser/serializing-html-fragments.html

<!DOCTYPE html>
<link rel="help" href="https://html.spec.whatwg.org/multipage/parsing.html#serialising-html-fragments">
<script src="/resources/testharness.js"></script>
<script src="/resources/testharnessreport.js"></script>
<body>
<div id="container"></div>
<script>
test(() => {
  class MyParagraph extends HTMLParagraphElement {}
  customElements.define('my-p', MyParagraph, { extends: 'p' });

  let p = new MyParagraph();
  p.setAttribute('class', 'foo');
  assert_equals(p.outerHTML, '<p is="my-p" class="foo"></p>');

  let container = document.querySelector('#container');
  container.appendChild(p);
  container.innerHTML = container.innerHTML;
  assert_not_equals(container.firstChild, p);
  assert_true(container.firstChild instanceof MyParagraph);
}, '"is" value should be serialized if the custom element has no "is" content attribute');

test(() => {
  let p = document.createElement('p', { is: 'your-p' });
  assert_equals(p.outerHTML, '<p is="your-p"></p>');
}, '"is" value should be serialized even for an undefined element');

test(() => {
  class MyDiv extends HTMLDivElement {}
  customElements.define('my-div', MyDiv, { extends: 'div' });

  let div = document.createElement('div', { is: 'my-div' });
  div.setAttribute('is', 'foo"bar\n');
  assert_equals(div.outerHTML, '<div is="foo&quot;bar\n"></div>');
}, '"is" content attribute should be serialized even if the element is a customized built-in element');
</script>
</body>