WordPress/wp-includes/blocks/details/style.css

.wp-block-details{
  box-sizing:border-box;
  overflow:hidden;
}

.wp-block-details summary{
  cursor:pointer;
}

.wp-block-details>:not(summary){
  margin-block-end:0;
  margin-block-start:var(--wp--style--block-gap);
}

.wp-block-details>:last-child{
  margin-bottom:0;
}