[webcomponents] Styling of elements where attachShadow(...) not allowed (#376)

Question started in #110, which is related, but since #110 is closed now, better move conversation here.
The main issue is that after `/deep/` was deprecated and some native elements are disallowed to have Shadow Root it becomes practically impossible to extend those native elements, because styling is one of the key features during `button`, `progress` or `input` extension.

There was few ideas in comments of linked issue, but the last (which is a compromise to avoid complicating things too much) is to state in specification something like following:
> Shadow Root in following elements \_list_of_elements_here_ MUST contain `<shadow></shadow>` in the root and MAY contain any number of `<style>...</style>` elements for styling purposes, any other elements are NOT allowed.

`_list_of_elements_here_` is basically blocklist of elements where `atttachShadow(...)` was not allowed. Effectively this only allows to inherit native Shadow DOM entirely and only add some styling to it.

Hopefully, some solution will be found till v1 release, re-implementing native semantic and work on accessibility for those elements is tricky, there should be a way to inherit all that goodness.

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Reply to this email directly or view it on GitHub:
https://github.com/w3c/webcomponents/issues/376