A page ships server-rendered declarative shadow DOM, then later loads the custom element definition. When the element is upgraded, what happens to the already-attached declarative shadow root, and how does the constructor's `attachShadow` behave?
class HostEl extends HTMLElement {
constructor(){
super();
// declarative shadow root already attached by parser
this.attachShadow({ mode: 'open' });
}
}
customElements.define('host-el', HostEl);