The CSS shadow parts module defines the ::part() pseudo-element that can be set on a shadow host. Using this pseudo-element, you can enable shadow hosts to expose the selected element in the shadow tree to the outside page for styling purposes.

By default, elements in a shadow tree can be styled only within their respective shadow roots. The CSS shadow parts module enables including a part attribute on <template> descendants that make up the custom element, exposing the shadow tree node to external styling via the ::part() pseudo-element.