alt Hacker NewsOne drawback of details was that cmd+f search wouldn't play nicely when the details was closed. But now there's a hidden="until-found" you can put on child content, along with an associated event. So you can open the details when a user searches :) super useful
You don't need the hidden="until-found" for details/summary, because that has those semantics automatically, but you can use that for other elements that behave similarly (for example tabs, which can't quite correctly be implemented with details/summary, and so needs to be done by hand).
Also I think the event isn't currently emitted consistently on all browsers (and maybe not at all for hidden="until-found"?) so unfortunately you can't quite rely on that yet if you need to sink some JavaScript state to your html. But in general, yeah, this is a really cool feature.