Chrome Version: ToT
OS: All but iOS

What steps will reproduce the problem?
(1) In <script> in an SVG document, run the following code:

  class MyElement1 extends HTMLElement {}
  customElements.define('my-element', MyElement1);
  let element = document.createElement('my-element', {});
  alert(element instanceof MyElement1);

  class MyElement2 extends HTMLDivElement {}
  customElements.define('my-div', MyElement2, { extends: 'div' });
  element = document.createElement('div', { is: 'my-div' });
  alert(element instanceof MyElement2);

What is the expected result?
Two alert dialogs with "false"

What happens instead?
Two alert dialogs with "true"

Please use labels and text to provide additional information.
Our Document::creteElement(local_name, string_or_options, exception_state) doesn't have a check for HTML-lie document.  i.e. "IsXHTMLDocument() || IsHTMLDocument()"

Firefox works correctly.