[w3c/webcomponents] Empty innerHTML in connectedCallback, but content is (#903)

Sorry for possibly duplicating issue, please redirect to existing discussion (parsedCallback or etc.).

Just wanted to point out to obscure API aspect.

```html
<script>
  customElements.define('x-x', class extends HTMLElement {
    connectedCallback() {
      console.log(this.innerHTML)
    }
  })
</script>

<x-x>123</x-x>
```

The element is rendered in DOM as `123`, but `innerHTML`/`textContent` is empty. Is that rendered/API state mismatch correct?
Expectation: it should either return `innerHTML` with factually rendered content, or if `innerHTML` is empty then it should not render `123`.
Current behavior blocks some practical solutions.

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