Re: [webcomponents] Styling of elements where attachShadow(...) not allowed (#376)

Thanks. BTW, one of the main reasons we removed /deep/ is it's too powerful and it's very unfriendly combinator in terms of the performance.

If we disallow any combinator, we could have the following declarative rules without siginificant performance penalty:

```css
@global-compound-selector {
 /* This rule applies to any element, even it is in a descendant shadow tree.
  However, we can not use any combinator in a selector here. */
 input {
     border : 10px solid black;
     color  : red;
 }

 input([disabled]) {
     color : blue;
 }

 input([compact]) {
     width : auto;
 }

 input([full-width]) {
     width : 100%;
  }
}
```

Just as a naive idea.

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