Using adoptedStyleSheets in custom elements

[jfalxa]

Describe the problem

I'm playing around with the svelte custom element API, and I was wondering if instead of inlining a new style element to every shadow DOM instance, you could use the shadowRoot.adoptedStyleSheets property.

Describe the proposed solution

Basically, it requires creating one instance of a CSSStyleSheet per component, setting its content with the component's CSS, and then referencing this single sheet inside the adoptedStyleSheets array of every new instance of the custom element.

https://developer.mozilla.org/en-US/docs/Web/API/Document/adoptedStyleSheets

Importance

nice to have

[dummdidumm]

Closing as duplicate of #8988