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It's not clear from the specification if any node (like text/comment node) can be an insertion point for shadow or Element node only. The only text that contains node type info is "The elements that host shadow DOM subtrees are called shadow hosts, and the root nodes of the shadow DOM subtrees are called shadow roots". But this text does not asserts that only Element nodes can be shadow hosts. I would propose asserting this directly.
http://dvcs.w3.org/hg/webcomponents/rev/b7b6c1795048