Shadow DOM is a set of JavaScript APIs for attaching an encapsulated "shadow" DOM tree to an element — which is rendered separately from the main document DOM — and controlling associated functionality. In this way, you can keep an element's features private, so they can be scripted and styled without the fear of collision with other parts of the document.
By default, Lit use the Shadow DOM to render your HTML markup. This is why your css style will be scoped to markup defined inside your component and won't affect other tags of the HTML document.