Discovery: using :host for low-specificity styles

[dvoytenko]

Related to #12.

In theory, a thing style-only-shadow could work for low-specificity styles on DOM level.

E.g. consider the following structure borrowed from the amp-selector spec. All of the contents of the amp-selector are in the light DOM. But the runtime could inject light shadow DOM subtrees with styles.

<amp-selector>
  <div option="value1" selected>
      #shadow-root          <-- this is inserted by the framework
        <slot></slot>         <-- propagate all DOM out
        <style>
           :host([selected]) {        <-- :host will have a low specificity
              outline: 1px solid blue;
           }
           :host:hover {
              outline: 1px solid lightgray;
           }
        </style>
  </div>
</amp-selector>

For more complicated trees, ::slotted can be similarly used.

Nuances to consider:

1. Performance of adding shadow root where it's otherwise not needed. Adoptable stylesheets could help a lot.
2. The :host() browser support.
3. Other costs.