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Nested DIVs also describe a tree but apparently that's too difficult to do with lambda calculus.

Tree wont' be a good fit for comments because one would want to read content by line/word/etc. while a tree item is presented as a whole thing by the screen reader so it's only good for small textual items.

There is also the aria-expanded role which could be added to a button if this [] were only changed to a button...