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

But from practical perspective it is useless. `div` and `span` have nothing special in them (no features, no semantic), so there is no real benefit to extend them or have Shadow DOM in them for other reasons.

What I really worry about is inability to style elements that extends build-in elements.
For instance, currently I have `input[is=cs-input-text]` and `progress[is=cs-progress]` elements that have some advanced features I need and expose some CSS variables/mixins. I do not need real Shadow Root, what I really need is just styling. Currently I achieved this with `html /deep/ input[is=cs-input-text]`, but since this is deprecated already I do not see any replacement, which is very sad (I do not want to re-implement all semantic and accessibility features of mentioned and some other elements).

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https://github.com/w3c/webcomponents/issues/110#issuecomment-148618729