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

These new pseudo classes splayed out across multiple comments are very confusing.
Let me try to summarize:

1. `:host` / `:shadow-host` - style the host of a shadowroot from inside the shadowroot
2. `:has-shadow-root` - matches any element in the "current" scope that has a shadowroot
3. `:potentially-custom` - matches any element that has a dash in the tagname?

Does that sound right?

If so, I don't think there's a real usecase for `:has-shadow-root`. It is unlikely that an author would mean to use anything other than `display`, as the selector is very broad. In fact, it is far more likely that the author would rather encapsulate the display styling either inside the shadow root with `:host` or perhaps another option to `attachShadowRoot` that can set the default styling.

Would `:potentially-custom` only match custom element candidates that don't have a definition? That could potentially be useful for styling lazily-defined elements to speed up first paint. If it always matches custom elements, I'm less optimistic of its utility.

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