:open
Targets elements that are currently in an 'open' state, such as a <details> element that is expanded or a <dialog> that is visible.
Quick example
details:open {
/* Adds a top border to the content box only when the details are open */
border-top: 2px solid #3b82f6;
}:open Browser Support
Limited availability 71% global usage
This feature is not Baseline because it does not work in some of the most widely-used browsers.
Not ready for production without a fallback.
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