Re: [webcomponents] Disallow attachShadow(...) for some elements. (#110)

I was thinking about another approach. It is a bit tricky to explain, but more flexible.

What if we support Shadow Root creation on all elements, but with blocklist which contains elements that might not change their *internal structure*. What I mean is that for elements like `input` or `progress` we will be able to add Shadow Root and place some styling inside in order to create custom-looking `input[is]` or `progress[is]`, but eventually it will not affect how they are constructed in terms of current internal pseudo-elements (I mean `::-webkit-progress-bar`, `::-webkit-progress-value` and others).

This is a bit tricky, but spec already contains similar tricks like self-closing HTML tags which are basically list of those special elements and everything else works by general rules. It would be really sad if styling postponed to v2 of this spec.

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