Re: [w3c/webcomponents] [idea] Allowing custom element registrations on per-shadow-root basis. (#488)

This would make it tricky to use `innerHTML` on a random `div` element that hasn't been connected yet, and therefore `querySelector` would not be able to know what tag names are associated with any of the elements.

Idea: what if the elements created with `innerHTML` et al are just inert until they exist in a live node tree within the document or within a shadow tree, and are only ever upgraded at that point? So,

```js
let div = new HTMLDivElement
div.innerHTML = "<my-form></my-form>" // MyForm is not instantiated because we don't know what registry is associated yet.
div.querySelector('my-form') // HTMLUnknownElement instance (or similar)
someShadowRoot.appendChild(div)
div.querySelector('my-form') // MyForm instance, has been upgraded based on root's registry.
someShadowRoot.removeChild(div)
div.querySelector('my-form') // HTMLUnknownElement, and MyForm instance was GCed?
```

Downside would be a loss of the MyForm instance in that example. Maybe there's other ways to make it happen? I can think of some nice ways, but they break backwards compatibility. For example, perhaps `querySelector` only works on "live" nodes that are in document or shadow tree, but then that means `querySelector` would not work on trees that haven't been attached yet.

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