A shadow tree is a tree of DOM nodes whose topmost node is a shadow root; that is, the topmost node within a shadow DOM. A shadow tree is a hidden set of standard DOM nodes which is attached to a standard DOM node that serves as a host. The hidden nodes are not directly visible using regular DOM functionality, but require use of a special Shadow DOM API to access.
Nodes within the shadow tree are not affected by anything applied outside the shadow tree, and vice versa. This provides a way to encapsulate implementation details, which is especially useful for custom elements and other advanced design paradigms.