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deepContains doesn't really explain what it does, and we could probably just, if needed, extend Node.contains to cover the use case.
I agree. It is very bad that we leave it in the spec while we are discussing a better alternative in https://www.w3.org/Bugs/Public/show_bug.cgi?id=22141. Let me get rid of it.
Removed in https://github.com/w3c/webcomponents/commit/463916e5873e99e43136ef4fe7e75b4da4c722aa