Re: [WICG/webcomponents] [scoped-registries] Interaction with declarative shadow DOM (#914)

> This might be somewhat surprising consequence of start using a scoped custom element registry and may cause havoc for some websites / web apps with a large number of web components with complex dependencies.

A web component author today currently has to design their components to work no matter the order that global-registry elements are upgraded. 

F.e., an author needs to make sure all of these permutations work:

```html
<script src="parent-el.js"></script>
<script src="child-el.js"></script>
<parent-el><child-el></child-el></parent-el>
```

```html
<script src="child-el.js"></script>
<script src="parent-el.js"></script>
<parent-el><child-el></child-el></parent-el>
```

<details>
  <summary>more permutations...</summary>

```html
<script src="parent-el.js"></script>
<parent-el><child-el></child-el></parent-el>
<script src="child-el.js"></script>
```

```html
<script src="child-el.js"></script>
<parent-el><child-el></child-el></parent-el>
<script src="parent-el.js"></script>
```

```html
<parent-el><child-el></child-el></parent-el>
<script src="parent-el.js"></script>
<script src="child-el.js"></script>
```

```html
<parent-el><child-el></child-el></parent-el>
<script src="child-el.js"></script>
<script src="parent-el.js"></script>
```

... etc ...

</details>

For any CE author that already designs their elements robustly, the load order differences will not be a problem.

**The top-down order is very intuitive and will lead to web developers having an easier time with less upgrade order issues. In React, Vue, Svelte, etc, the order is always top-down. It would confuse people otherwise.**

The _ease_ of the developer experience may be something to consider here.

The _single most difficult_ part of Custom Elements (for me) has been dealing with upgrade order.

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