At the time of this posting, <details> and <summary> tags aren't listed on Dreamwidth's Supported HTML FAQ entry. However, I've been chatting with denise about this, and this is not indicative of the tags' support being an unintentional bug likely to be patched out; rather the FAQ entry is just outdated (and, now that I've pointed out the issue, set to be updated at some point). So there's no need to worry about support for the tags being suddenly revoked.