[webcomponents] Use a parent/child relationship in the flat tree in the definition of some elements, i.e. <tr>/<td> (#404)

See the context: https://www.w3.org/Bugs/Public/show_bug.cgi?id=28353.
I do not think we need to address this issue for v1. "It should not work by default" might make sense for a while.

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Reply to this email directly or view it on GitHub:
https://github.com/w3c/webcomponents/issues/404