How is that relevant to CSS?

It is actually relevant: https://developer.mozilla.org/en-US/docs/Web/Web_Components/...

Support for it only exists on Chrome at the moment (https://caniuse.com/#search=shadow%20dom), but things like Angular are already geared up to use it for encapsulating styles, but defaulting to an emulated mode that is essentially css-modules: https://angular.io/guide/component-styles#view-encapsulation

Vue scopes components through data attributes. I'm not entirely confident we really need shadow dom support in browsers, as we have good strategies now of creating components without it.

Safari and Opera support shadow DOM, Firefox is adding it currently.

Interesting, thanks.