Re: [w3c/webcomponents] Provide a lightweight mechanism to add styles to a custom element (#468)

### [Please see this issue on construct stylesheets](https://github.com/WICG/construct-stylesheets/issues/2).

@rakina I can't say this enough, based on the discussion in the constructible stylesheet proposal the `CSSStyleSheet` constructor _will not_ take an argument because the CSS working group wants to keep parsing asynchronous because vendors apparently want to parse in separate threads (Firefox already does this).

I feel like the conversation in this thread is continuing without regard for the decisions/discussion made elsewhere and that will have a profound negative impact on this feature. The assumptions made here aren't congruent with the assumptions made elsewhere.

The way I see it there are several options:

- Allow the style attribute on a custom element config object to accept `CSSStyleSheet` or `Promise<CSSStyleSheet>`.
- Allow for modifications of the stylesheet after it has been added (which you have already said is possible).
- Expose some feature on the `window.customElements` object that allows for the getting and modification of custom element default stylesheets (example: `customElements.getElementStyleSheet(SomeElementClass)` which would return the `CSSStyleSheet` object.
- Some combination of the above.

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