Update to support revamped proposal

[sorvell]

Fixes: #603

New proposal: whatwg/html#10854

[sorvell]

To prevent future issues with conflicting with native implementations, let's consider offering a version of this that's more of a ponyfill. For example:

pony(document).createElement(tag, {customElements})
pony(element).customElements
pony(element).attachShadow({mode: 'open', customElements})

This might be fairly annoying in practice and there are some options:

1. ponyfill or polyfill HTMLElement and CustomElementRegistry?
2. ponyfill or polyfill relevant APIs, which include:
   • Node.customElements
   • Node.prototype.appendChild
   • Node.prototype.insertBefore
   • DocumentFragment.prototype.append
   • ShadowRoot,.prototype.setHTMLUnsafe
   • ShadowRoot.prototype.innerHTML
   • Element.prototype.insertAdjacentHTML
   • Element.prototype.setHTMLUnsafe
   • Element.prototype.append
   • Element.prototype.prepend
   • Element.prototype.insertAdjacentElement
   • Element.prototype.replaceChild
   • Element.prototype.replaceChildren
   • Element.prototype.replaceWith
   • Element.prototype.innerHTML
   • HTMLElement.prototype.attachShadow
   • HTMLElement.prototype.attachInternals
   • Document.prototype.createElement
   • Document.prototype.createElementNS
   • Document.prototype.importNode

[justinfagnani]

It'd be nice for reviewing and future readers to leave links to where public APIs are specced. I think this link would be good here: https://dom.spec.whatwg.org/#dom-documentorshadowroot-customelementregistry

[justinfagnani]

Maybe choose a more descriptive name or add a comment. Maybe setRegistryForSubtree?

Also, does this implement a spec operation? If so, it'd be useful to have a link.

[justinfagnani]

what about initializing a non-shadow root parent, or initializing a subtree where some elements already have registries?