This area is a bit hairy. The spec only has Document (HTMLDocument is no more) so the wrappers only have Document. No matter oif the native document instance is an HTMLDocument or Document the wrapper we create is always Document.

https://github.com/arv/ShadowDOM/blob/07be1f387f9a099bd9a13a0edf193acbf0f5c522/src/wrappers/Document.js#L133
```js
  // Both WebKit and Gecko uses HTMLDocument for document. HTML5/DOM only has
  // one Document interface and IE implements the standard correctly.
```