was just doing some email/github housekeeping, and checked in on this issue i filed back in 2017... "Ability to style <details>/<summary> to be open (e.g. for print styles)" https://github.com/w3c/csswg-drafts/issues/2084 (98 thumbs-up? oh my!)

had missed that this was actually taken up and implemented (see last comments on that issue)

/cc @aardrian (on the very rare offchance that he might not have seen this)