Heads Up! It's all about the V1 Spec.
In a nutshell, Shadow DOM enables local scoping for HTML & CSS.
Shadow DOM fixes CSS and DOM. It introduces scoped styles to the web platform. Without tools or naming conventions, you can bundle CSS with markup, hide implementation details, and author self-contained components in vanilla JavaScript. - https://developers.google.com/web/fundamentals/getting-started/primers/shadowdom
It's like it's own little world which hardly affects or gets affected by the outside world.