Re: [w3c/webcomponents] Support Custom Pseudo-elements (#300)

Hi @tabatkins, I like this proposal a lot!  It seems that there is some overlap with the `@apply` proposal here. Where would you see this being useful where `@apply` would be less useful?

One thing that comes to mind, and correct me if I'm wrong here, is that `part=` makes it even a little easier to theme that `@apply` mixins do, because you could cascade styles more easily. For example a CSS framework might tell you to add a part attribute to your form like:

```html
<form part="bootstrap-form"> .. </form>
```

And then they could style anything within the form like:

```css
:root::theme(bootstrap-form) label {
  /* special label rules */
}
```

Whereas you couldn't get this specificity using `@apply`. Is that the idea?

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