A shadow part is a named element in a shadow tree that exposes a controlled styling hook through CSS ::part. It addresses a practical limitation of Shadow DOM: ordinary styles cannot cross the shadow boundary, which can otherwise prevent an application-wide design system from styling component internals. section 15.1
Shadow parts connect component encapsulation with modular design-system workflows. They let reusable components retain internal structure while making selected styling surfaces available to applications, frameworks, and themes. Their value is especially apparent when CSS is maintained as small, modular source files rather than one application-wide stylesheet. section 15.1
