The shadow root is attached to another node tree through a specific DOM node referred to as its host. This host may be part of another shadow tree or part of the regular DOM tree. The node tree of a shadow root's host is sometimes referred to as the light tree.
The hidden DOM nodes of a shadow tree are generally not affected by anything applied outside the shadow tree, and vice versa. The shadow boundary, where the shadow DOM ends and the regular DOM begins, can be traversed, but only very intentionally: