Problem/Motivation
This is part of the CSS modernization initiative.
Steps to reproduce
Remove @nest from the stylesheet present at https://git.drupalcode.org/project/drupal/-/blob/11.x/core/themes/claro/css/components/details.pcss.css
Proposed resolution
@nest was removed from the specification completely. Future versions of postcss-nesting plugin will error if you use @nest.
Refer to this link for further details.
Remaining tasks
Provide before and after screenshots.