docs: function isCustomElement(element) {
    assertType(element, Node, false, 'Invalid element specified');
  
    let is = getAttribute(element, 'is');
    let tag = element.tagName.toLowerCase();
  
    if (is && (getElementRegistry(is) !== undefined)) return true;
    if (tag && (getElementRegistry(tag) !== undefined)) return true;
    return false;
  }