Lack of [[PlatformBrand]] is biting us #597
Description
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).