I noticed that I couldn't get flexbox to work for a <details>/<summary> pair. Emilio pointed to the spec which says:

The details element is expected to render as a block box.

which ends up meaning that (cross-browser) you cannot change its display value to flex. Emilio suggested this may change in the future, and we could experiment with it behind a pref.

David, this is the "<details> is always a block" bit that I mentioned when you brought up details styling. Do you know if there's an issue on the HTML spec to remove that requirement or if that's part of your work for <details> improvements?

This is probably something we want to move towards, so it doesn't hurt
adding a pref to allow people to experiment with it.

I'm not aware of an issue against the HTML spec on this.

I am interested in working on fixing this, as discussed in the CSS WG meeting in July. I've prototyped this change and some other changes in Chromium in https://crbug.com/1469418 , but with the intent of experimenting with what I prototyped and likely making major changes. I got a bunch of feedback from Bramus who tried to use the prototype to build a bunch of things with it, and I'm part of the way done going through that feedback. Once I do that, I need to (possibly not in this order) write some major updates to https://github.com/dbaron/details-styling, probably try to get another round of feedback from Bramus and hopefully build a few demos, and start making some more concrete proposals for what to change in some combination of the Open UI CG / the CSS WG / the HTML spec. I'm hoping to get back to working on that sometime this month (but I've spent the past month mostly doing two other things, one of which is <details name>).

https://hg.mozilla.org/mozilla-central/rev/4416f2eba0ce