[whatwg/dom] Lack of [[PlatformBrand]] is biting us (#597)

Consider https://dom.spec.whatwg.org/#concept-node-ensure-pre-insertion-validity. Step 4 bans `DocumentFragment`. Does it ban `ShadowRoot`? If it does, that's fine, but also a problem, since it shouldn't ban `CDATASection`.

I thought we had an issue tracking this, but it seems like we don't.

This is somewhat related to #457, which is about updating various algorithms to account for `Attr` nodes, which can now be passed to them again (and not all are anticipating that).

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