Web interfaces contain several elements that can be open or closed: disclosure widgets, dialogs, select pickers, date inputs, and color pickers. Historically, styling those states required different selectors. A disclosure could use details[open] , while native picker state often had no equivalent general selector. The CSS :open pseudo-class provides one semantic selector for elements that are currently in an open state. MDN marks it Baseline 2026 Newly available since May 2026 across t...