Re: [webcomponents] Sharing styles across custom elements (#282)

The shadow-piercing combinators are indeed helpful, but they're also (a) terrible for performance, and (b) footguns, as you can easily style things you didn't intend.

@Nevraeka is right - using Custom Properties (soon to be released in Blink) as the theming mechanism is safer and more targeted, and more performant.  We expect to add more theming abilities in the future, like perhaps a way to expose an element/subtree from your component as a pseudo-element on the host, so you can do arbitrary styling.  But that's in the future, and is just a convenience measure.

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