Re: [whatwg/dom] Declarative Shadow DOM (#510)

I'll echo @treshugart's point that for React, the shadow DOM is only overhead and complexity, when the only thing we'd really want out of this is scoped CSS. If we can't have that, we could work with a shadow DOM too. I'm sure @wycats had thoughts on this too.

I would expect many trees to result in code like:

```js
<div>
  <template attach-shadow shadow-mode="open">
    <div>
      <template attach-shadow shadow-mode="open">
        <div>
          <template attach-shadow shadow-mode="open">
            <div>
              <template attach-shadow shadow-mode="open">
                ...
              </template>
            </div>
          </template>
        </div>
      </template>
    </div>
  </template>
</div>
```

Since that's semantically what a modular composed tree would be like. We'd just treat the shadow tree as the new children tree. Anything else would be an optimization (which might need to bail out).

What would be *very* interesting, however, is if you could target a particular ID that was defined earlier to attach the shadow lazily later in the document stream.

```js
<section>
  <h1>Heading</h1>
  <p id="content">Loading...</p>
   ...
</section>
...
<template attachto="content">
  Hello world!
</template>
```

Even then, I'd prefer it if it replaced the children with the content of template, but we could live with the workaround to use shadow DOM.

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