Nope. I was as surprised as you are when I tested CPCBox on IE9 beta. But the truth is that Javascript support in Internet Explorer has been freezed on version 1.3.
Microsoft has since switched to an edulcorated language called "JScript".
That thing is technically ECMAScript compatible but is in fact just another case of "Embrace, Extend and Extinguish technology". Needless to say that no other browser supports this JScript stuff.
Another approach to please IE would be to backport CPCBox to Javascript 1.3.
The main difference between Javascript 1.3 and 1.5 for CPCBox is the support of const declarations. These const could be switched to var declarations easily to be Javascript 1.3 compatible. The downside would be some performance impact. Not nice given how slow CPCBox currently is :/
But the point is I could have backported CPCBox to Javascript 1.3 to please IE. And in fact, I was really considering doing just that.
But then I stumbled upon another issue with IE. It doesn't support binary strings.
It's big issue because in Javascript, files can only be retrieved from the server as strings.
To workaround this, it would be necessary to convert binary files (ie: CPC snapshots) to Base64 encoded strings to store them on the server. And then decoding Base64 strings with Javascript on the client to retrieve the binary data.
To keep it short, pleasing IE9 means lots of specific code and dedicated effort to workaround its deficiencies and it's not worth it for a spare-time project.
To put the nail in the coffin, IE9 Javascript performance is not exactly stellar. From my own tests, it's around 30-40% the speed of Chrome 6. So it's not like CPCBox would be really usable in IE9 anyway.