They both return U+00BF on Windows. On a Mac nightly, "?" gets U+003F and "/" gets U+002F. Firstly, Windows and Mac should agree here? And second, / and ? should get different values. Also, I think the DOM events spec was modified that the keyIdentifier here actually should be "?" and "/".
The editor's draft of DOM 3 Events no longer has keyIdentifier. If we can't remove it, perhaps we should keep its existing behavior, no matter how little sense it makes. On the other hand, having platform differences is not great.