Shadow parts ::part()
The way a web component nominates which of its internal elements may be styled from outside. An explicit contract rather than holes in the encapsulation.
The entire point of the shadow tree is that outside CSS cannot reach in, but taken literally that makes a component impossible to fit to a design — which is why early component libraries either abandoned shadow DOM altogether or exposed fifty custom properties for every conceivable attribute. The solution is that the component author marks an internal element with part="label", after which the consumer writes my-widget::part(label) and styles it normally. The important thing is the limitation: only what has been explicitly exposed is reachable, and only as a whole element — you cannot select further down into its children. The exportparts attribute forwards parts up through nested components.