files/en-us/web/css/guides/shadow_parts/index.md
The CSS shadow parts module defines the {{cssxref("::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 {{HTMLElement("template")}} descendants that make up the custom element, exposing the shadow tree node to external styling via the ::part() pseudo-element.
{{Specifications}}