None of the portable readers support all the potential ePub specs; CSS allows a lot more flexibility than the software & hardware on the readers. (Hardware: CSS allows color; e-ink readers aren't going to show that correctly; some readers don't show some image types, regardless of whether they're properly coded.)
We don't yet have enough consistency in support to need an acid test--we're still at the "this one doesn't do justification" and "that one doesn't allow cross-document links"* stage.
(*Hypothetical. I have no idea how cross-document linking in ePubs works.)
Irrelevant side note: Firefox 3.6.3 gets me 92/100; the blue box shows up as grey. Internet Explorer 8 gets 20/100, doesn't finish loading at all, and shows error messages.