Internet Explorer 5.5 was the default pack-in browser with Apple's OS X at time of writing. Microsoft worked really hard to Carbonize their build of the browser to get it ready for Apple's moving release target. The browser is fast, efficient, and is almost a model Carbon citizen.
Macintosh IE is a completely different codebase than IE for Windows. Mac IE is based on a rendering engine known as Tasman, while the PC engine is based on an engine known as Trident.
Mac IE 5.5 was developed not in Redmond, but in Silicon Valley, and because of this, the team is very close to Apple in Cupertino, California, and can consult with each other while trying meet each other's expectations. I hear it was a good project to be on, and one that was mutually beneficial for both parties.