The ::picker() pseudo-element targets the picker part of a form control, that is, the pop-up part that appears to allow you to make a selection when you press the control button. It is only available to target when the originating element has a picker and has base appearance set on it via the appearance property base-select value.
The ::picker(select) selector targets all descendants of customizable <select> element except for the first <button> child; these descendants are grouped together by the browser and rendered as the picker. The first <button> child represents the control button that opens the picker when pressed.
This allows you to target all of the picker contents as a single entity, for example if you want to customize its border, animate it when it appears and disappears, or position it somewhere different to the default position. Our customizable select elements guide shows many examples of ::picker(select) usage.