Splitting this up into two different things is great. Allowing attaching an alternative shadow tree through CSS but disabling any JS to be run seems like the right thing to do.
I'm also in favor of the "is" attribute. Even though I think that <x-foo> is more readable than <div is="foo"> it is hard to argue about the issues regarding parsing and fallback behavior. The "is" attribute also provides a better behavior for common cases such as when you want to extend <select> and <button>. erik
