Re: [webcomponents] Should shadow host have `display: block` by default? (#426)

A couple of thoughts:

- It would upset authors if we attach a shadow root to `<span>`. Authors would expect `<span>` is still 'display: inline'.
- `:host` is a relatively cost-expensive selector because it is one of cross-scoping selectors. It's defined in a child tree, but it is applied to a shadow host in a parent tree.
  
How about having another pseudo class, like `:custom-host`?
That is applied to an element if it is a custom element and a shadow host.


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