Re: [shadow-styling] clarify role of the `active shadow tree`

Upon further reflection, I think it would be better if the `active shadow
tree` and the <shadow> element had *no influence* on whether or not a
stylesheet applies.

Instead, all stylesheets in all sibling shadowRoots should always apply.
For rules without :host, there would be no detectable change since the
styles are still scoped to individual shadowRoots. However, there are
implications for the :host selector since it reaches up to the host
element. In particular, this means if there's a :host rule in an older
shadowRoot, it always applies to the host element.

I believe 90% of the time, this will be what the author wants. In the case
where it's not, the onus is on the author to override the :host styling in
the younger shadowRoot. This seems like a good tradeoff.