https://developer.mozilla.org/en-US/docs/Web/CSS/Guides/Shadow_parts
CSS shadow parts - CSS | MDN
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...
shadow partscssmdn
https://www.w3.org/standards/history/css-shadow-parts-1/
CSS Shadow Parts Module Level 1 publication history | Standards | W3C
The World Wide Web Consortium (W3C) is an international community where Member organizations, a full-time staff, and the public work together to develop Web...
shadow partslevel 1cssmodulepublication