:openThe :open CSS pseudo-class matches elements that have open states, like <details>, <dialog>, or <select>, based on their state.selectorsSelectorsnewly2026-05-11https://drafts.csswg.org/selectors-4/#open-statehttps://developer.mozilla.org/docs/Web/CSS/Reference/Selectors/:opencss.selectors.openhttps://nowyoucanuse.com/features--open-pseudo.html