The current proposal for exclusive accordion proposes that exclusivity is only enforced for details elements whose root is a Document or ShadowRoot. This has been discussed in the HTML PR for adding the feature, and based on the discussion there, I wanted to raise it explicitly here.

I made this decision because it seemed less constraining on implementations: in particular, if implementations are going to cache information about which details elements are in the same group, they need to store that cache somewhere, and that location makes sense as DocumentOrShadowRoot. It was pointed out that this behavior is inconsistent with radio buttons, however @emilio pointed out that the radio button behavior has known bugs in current implementations, so it's not perfect.

In the current Chrome implementation (since the change to do notifications in tree order), I'm actually not taking advantage of this behavior, so it would be entirely straightforward to switch. However, it would make it harder to reintroduce any form of caching in the future if we decided it was useful to optimize for speed.

So I'm interested in feedback on this decision. And most specifically, I'm interested in feedback from developers or those who understand their needs as to how useful having this feature work in subtrees whose root is not a Document or a ShadowRoot would be (for example, when setting up something to be inserted later).