This is good enough to make me consider using it even with the browser compatibility issues involved

Brilliant. What's important, with simply conditional CSS it's still usable with older IEs (ie. not totally broken). The collapsing functionality can be considered as optional usability.

I don't know...IE9 isn't close to being released is it?

It's about to enter public beta... release is probably at least a year out.

You'd have to browser-check and then load something like the jsTree stuff if it failed. That'd be a major pain.

But the performance and styling is looking really good. It's very tempting.

Sorry to nitpick grammar, but this sort of stood out:

"Not contempt with any of those solutions I investigated doing it with pure CSS..."

(contempt -> content)

Ah thanks for that, it slipped by my proof reading.

Very nice. But--pardon my pedantry--there seems to be some HTML in this "pure CSS" tree menu.

Nothing about the HTML specifies this kind of menu styling.